I am new to clubwrx.net, and wanted to contribute something right away, which may be useful to a few people out there.
This guide explains, step by step, how to install an aftermarket head unit in your 02-03 (and possibly beyond) WRX. I tried to make this guide easy to follow, and included a lot of pictures (probably too many). Clicking on a thumbnail will enlarge the image to full size.
I am by no means an expert on this topic. If you feel there is anything I left out, anything that should be expanded on, or anything that is blatantly wrong, please feel free to point it out.
Step 1 - Getting started
The first thing you should do is gather all of the equipment and hardware you will need to perform this installation. Listed below is what I recommend, at a minimum:
-Philips head screwdriver
-Multipurpose crimper/wire stripper/wire cutter
-Wiring harness (Vehicle specific, should be available almost anywhere car audio is sold)
-Head unit wiring harness (Should come packaged with new unit)
-Din pocket (Fills hole in dash left by double-din stock unit)
Step 2 - Preparing the wiring
After you have gathered all necessary equipment, you are ready to begin the wiring process.
Start by exposing roughly a quarter inch of bare copper on each wire. You may wish to twist the ends together. This will make it easier to insert the lead into the connector.
Insert the exposed wire into the connector. Twisting the connector as you insert the wire will allow the wire to penetrate further, which will aid not only in making a better connection, but will help to hold the connector in place as you prepare to crimp it.
Once the connector is in place, crimp at the end where the wire is inserted, using the specified slot for your connector size on the crimping tool.
Squeeze as tightly as possible to ensure a good connection. If the metal tube within the connector does not clamp tightly enough around the wire, it may slip out if it is stretched taut.
Repeat the process with the other end of the wire, creating a completed connection between harnesses. After both connections are made, pull lightly on each wire. If it feels loose within the connector, you can create a second crimp next to the first to help hold the wire in place more tightly.
Repeat this process with all remaining wires. Be sure that you are connecting the right colors, as speaker connections will have two wires of the same color. The negative wire should always have a black stripe running the length of the wire, but if you are not paying attention, you could easily connect it to the positive wire on the other harness, as the primary colors are the same.
It is possible that you will have wires left over that you will not use, including the remote amp turn-on (this is likely a blue wire), dimmer (orange), and others. I recommend using electrical tape to cover the exposed leads on these wires. If you do not cover these ends, the wire could make contact with a metal surface within the dashboard and cause a short circuit.
Step 3 - Removing the stock unit
After you have completed all of your wiring harness connections, it is time to remove the stock unit from the car.
CAUTION!!! Always disconnect the negative battery terminal when working on electronics within the car. Failure to do so could cause a short circuit, frying your head unit, and possibly worse. If you choose not to heed this warning, do so at your own risk.
After the battery is disconnected, we can get to work removing the stock unit.
I am not entirely sure that this step is necessary, but I find it much easier to remove the additional trim by taking the cupholder out first. Eject the cupholder as if you were about to use it, and remove the 2 screws holding it in place.
With the cupholder out, we can remove the trim panel which surrounds the stereo and climate controls. The picture below marks the approximate locations of the fasteners which hold this trim in place. Beginning at the top right corner (where we removed the cupholder), pry gently to unsnap the trim.
With the trim removed, we can now get to the bracket which holds the stereo in place. Remove the 4 screws which lock the brackets to the dash.
After these screws are removed, the head unit should slide out of the dash with relative ease. Take care to disconnect the wires as you pull the head unit out, and try to avoid putting too much tension on them. The ground wire on the right bracket can be difficult to remove the first time (I had to use pliers to get it off).
Our stock unit is out. Time to mount up the new one!