I was unable to get this proof-read before posting. Please PM me with any errors rather than replying in-thread.
I'm writing this tutorial for kind of a strange reason: The install is very, very easy. Frankly, if you can rotate your tires, you can swap your lateral links. The problem is that no one seems to know it. The trailing link and its bushings are an often overlooked part of the suspension. This is especially criminal when you consider how amazingly easy it is to swap both the bushings and the links in comparison to how much the swap helps address one of the biggest problems with daily driving an Impreza (driveline bounce, especially a problem for the DBW cars). This tutorial covers just swapping the trailing links themselves as though it were the only thing being done. There's also a companion tutorial to this one describing how to swap the rear of trailing link bushing using the Big Sky Bushing Tool. If you're doing one, you probably want to combine both installs into one meta-install because they share many of the same steps. When reading these instructions, keep in mind that these basic steps apply to any trailing link swap, wither your replacing an bent one, installing an OEM link with pre-pressed Group N bushings, or upgrading to a race-spec carbon-kevlar spherical bearing unit. It's all the same, and it's all very simple.
By the way, clicking on any of the images will get you a larger, more detailed, image.
Start the install by jacking up the car and removing the rear wheels. The front wheels can stay on. They can stay on the ground, for that matter. Once you have the wheels off, crawl under the car from the back and look towards where the wheels normally are. You should see something that looks like this:
The lump hanging off the bottom of the knuckle is the rear of trailing link bushing. The trailing link extends from that bushing on the rear forwards to where it bolts to the chassis. We'll be removing the bolts at each end and replacing this link.
OK, now that you know what you're working with, get out from under the car and look into the wheel well just in front of the brake rotor. You'll see two lines bolted to the trailing link. They'll look like this:
We need to detach both of these lines from the trailing link before removing the link. There are two parts to the bracket. If you look at the previous picture, there's a hoop that sticks up on top of the link and a bolt farther up the link. Use a 12mm socket to remove the bolt on top of the link and then crawl back under the car to remove the bolt holding the hoop. Do this on both sides of the car. Since you can't see the second bolt from beside the car, here's a view from underneath, showing the bolt (note that your bracket might look slightly different depending on model year):
Next up is detaching the rear end of the railing link itself. Strangely, the nut is on the outside of the trailing link between the knuckle and the brake rotor. This makes it slightly tricky to find and to loosen. This picture is taken from the rear, showing you the bolt you'll be working on and how it hides behind the rotor:
The best way to remove the nut from this bolt is to use a pair of 6-point combination or box wrenches (NOTE: DO NOT TRY TO USE 12 POINT WRENCHES. If the nut is corroded in place, 12 point wrenches will very likely round off the nut) and a heavy (2.5lbs or greater) plastic dead-blow hammer. A socket and a breaker bar could be used on the head side, but would not fit on the nut side. Place one wrench on the head (inside of the bolt to prevent it from spinning. Place the other wrench on the nut (outside) in a position that allows it to be easily struck with the hammer. Strike the end of the wrench VERY HARD with the dead-blow hammer. This is not a time to be gentle and worry about breaking something. You need to be in full-on "Lothar SMASH!"-mode here. Keep beating on the wrench until you can spin the wrench by hand. After that, use a ratcheting wrench on the head side while holding the nut still to remove the nut. Now remove the bolt from the link. CAUTION: Depending on manufacturing tolerances, this bolt may be under considerable strain. It is possible that the link will spring away from the knuckle and/or that the knuckle will spring away from the link (taking the lateral links and strut with it) when you remove this bolt. Don't have you head in the way!
Once you have the rear end of the link free, follow the link up to where it bolts onto the chassis. You will see that there is a bolt holding the link into a bracket which is itself bolted onto the chassis. Ideally, you can get just the bolt holding the link to the chassis to come out. If this bolt is rusted solid, you can remove the bracket from the chassis with the link still bolted into it and then separate the link from the bracket by whatever means necessary. The following picture shows the view as you look up the link from the rear of the car. The bolt you'd like to remove the is the large one running from right to left in this picture:
The only complication to this entire install is the presence of the parking brake and other lines right in front of the bolt you want to take out. On 3 of the 4 cars I've done this to, I was able to remove the bolt by just bending the lines out of the way. On the 4th car (the one photographed for this picture), the bracket holding the lines in place had to be removed to create enough room to remove the bolt. The following picture shows how the bolt is hemmed in by lines:
You'll again be using the two 17mm, 6-point combination or box wrenches and the dead-blow hammer to remove this bolt. Once the bolt is removed, pull, yank, twist, wiggle, or whatever it takes to work the trailing link out of the bracket. Congratulations, you're half-way through the install!
Now grab your new link and make sure you have it facing the right way. For OEM links, this means with the open/hollow side facing in. For aftermarket links, follow the manufacturer's directions for determining the correct orientation. Wiggle the link up into the bracket until you can slide the bolt in. Place the nut on the bolt and tighten it finger tight. Do not tighten it with a wrench at this time.
Move to the rear of the trailing link and slide the link up and around the knuckle. It's very likely that the hole in the link and the hole in the bushing will not be aligned. You will need a friend to push or pull on the knuckle until the holes are aligned and the bolt can be slid home. On some cars, this will require a VERY large amount of force, so be sure you have the wheels securely chocked before you try to move the knuckle into alignment. Place the nut onto the bolt and again tighten it finger tight but no more.
Rubber has a property called hysteresis. For our purposes, this means that whatever position we tighten the bolts down in is the position the rubber bushings are always going to fight to return to. For a car, this obviously means we need to tighten them down at ride height. The best way to achieve this is to use a floor jack. Place a piece of 4x4 on the jack and raise it up under the knuckle. Make sure the wood and jack are positioned so that you can swing the wrench when you tighten the bolt (note that the picture below shows the jack in the wrong position for this install, you actually need to bring the jack in from the rear of the car). Raise the knuckle with the jack until the knuckle is at ride height or the chassis begins to lift off the jack stands. Note that the chassis must still have its weight resting on the jack stands for safety. The picture below shows the basic idea, but remember that the jack is in the wrong position in this picture, you'll need to bring the jack in from the rear:
Use a torque wrench and 17mm Crow's Foot to torque the rear bolt to 66lb-ft. Use the same setup to torque front bolt to 85lb-ft. Drop the jack slowly to let the knuckle down. Do this on both sides of the car.
Re-install the brackets holding the parking brake and other lines to the trailing links using the two small bolts. Torque these bolts to 22lb-ft.
Visually verify that everything looks correct and make sure you don't have any extra parts. Re-install the wheels using a star-shaped tightening pattern and torquing the lug nuts to 70lb-ft. Drop the car to the ground and take it for a test drive.