Just put one in
I just put a factory unit into my MY10 STI. Only took about 90min and all went well. Do you have the instructions that came with the unit? They are not bad but a couple of points in them need modification slightly IMHO... also, I did it without unbolting the seat (as that looked way too hard!).
So let me know if you have the instructions and I'll just tell you my tips. Otherwise I'll try to post or link to the instructions as well.