I just installed an 05+ STi steering wheel into my 02 WRX. was fairly straightforward. the main issues are airbag, horn and cruise control.
Here's some pics so you guys can have an idea what it all looks like in there:
02 WRX MOMO
02 WRX Airbag
02 WRX charge removed
STi charge removed
Charges from the top
Charges from the side. You can see how the dual stage charge is taller.
Fitment of 02 WRX charge into STi airbag. Okay, this was my solution. Have no idea if it would work or how properly. I figure, the single charge is enough to inflate enough of the bag in the event of something, right? Yeahhhh. Hopefully I won't ever find out. Knocks on wood.
But it makes wiring a piece of cake as it just plugs right in.
So next issue was the horn. No pictures for this. But currently I have simply stuffed a piece of wire into the horn spade in the plug of the STi wheel and into the plug of the car harness. Its the green wires on both wheel and car. Beep Beep. works fine.
Next will be cruise. i think i'll try the wire stuffing technique with that as well. see if it works. so far haven't had to cut a single wire.
Installed. I love this wheel. The smaller diameter is so much nicer.