This process was along, confusing, and frustrating one. I've been going through this for almost 2 weeks now, and just this afternoon, I finally figured it out. I've done SO MUCH searching.... here, NASIOC, even the local STL subary club..... and there was NOTHING on the appropriate way to install a tach or a shift light.
Lots of info about the infamous "green wire" on the center plug behind the gauge cluster. Which by the way... the wire on an 06 WRX is on the passenger side plug, and it's not green. Doesn't really matter though... that wire will "kinda" give youa signal, but it fluctuates, and its not reliable or accurate...... don't use it. I found this out the hard way.
I started with a C2 Autometer DPSS Stage 2 digital tach/shift light (part # 7188). I tore apart my dash for 3 hours..... installed it on the infamous "green wire"..... or in my case, the wire that I tracked down to be the tach wire on an 06...... and the readings were all over the place...... useless.
So... I called Autometer, and spent 45 minutes on the phone with them.. and FINALLY got directions on how to install it properly...
Oh yea.... AND YOU HAVE TO BUY THE AUTOMETER "TACH ADAPTER" TO GET IT TO WORK FOR $75.00.
Of course, they don't tell you that.... the directions aren't posted anywhere online.... you have to call and "take notes". Oh yea, and it doesn't help that they actually use the WRX to advertise their friggin product (check out the dashboard displayed on the Autometer website):
(click on "gauge"... then on "series")
So... I buy the stupid tach adapter (part # 9117).... and turns out... the directions they gave me were for an older WRX. Well... I'll post how to do it on an older WRX, as well as the 06s... just so no one else has to go through this crap like I did..... searching wires and what not.
(on the tach or shift light)
1) red goes to your AC power
2) black goes to ground
3) green goes to the GREY wire on the tach adapter
(on the tach adapter)..... (this is the confusing part)
4) Black goes to a GOOD ground (I grounded to the firewall)
5) Grey goes to the green wire on the tach (see step 3)
6) Red, and red w/green stripe..... both go through the firewall
7) There is a BROWN connector..... located by the driver's side cylinder head. The easiest way to get to it is to remove the battery.... you can't miss it. The brown one is on top, and theres another white one below it.
8) Disconnect the brown connector from the metal bracket....... then unclip the connector. The side you'll use is the side closest to the engine.
9) use a razor to remove a bit of the wire-wrap.... as you'll need access to a good 3 or 4 inches of wire.
10) There are 16 wires in this brown connector..... the one you want is the RED WIRE WITH THE YELLOW STRIPE.
(for you 2006 owners..... it's a solid RED wire. There are 2 of them, so you want the one that is on the same side as the safety clasp tab... closest to the ground. Its the same location on earlier models, but a different color)
11) Cut that wire..... allow at least 2 inches on both sides.
12) The red/green stripe wire from the tach adapter gets spliced onto the part of the wire you just cut..... closest to the brown pigtail
13) The solid red wire from the tach adapter goes to the other side of the wire we just cut.
(If you get #12 and #13 backwards... you'll get a "0" reading on your tach.... then just switch them around)...... yes... I did it.
14) Clean everything up... secure everything... put your battery back in, etc... your done.
The tach is sharp as......... well.... a tack I guess ) It works perfectly.... the RPM readings are DEAD-ON... does not hesitate, fluctuate or anything., My shift light is set at 6200 RPM for the first gear.... and 6450 RPM for every other gear.
I know that a lot of folks that have installed shift lights use them as a "guide" because they aren't accurate. I've never read a post about someone installing an aftermarket tach in a WRX. Because our cars have coil-on plug ignitions.... you can't get a good, useable tach signal from them...... unless you use the tach adapter, and wire it in correctly.
I do hope this helps a few folks out there.
Sorry for the crappy pic of my dashboard.. my camera is a piece of junk.
best of luck! --Matt (ninefourteener)