Well, I was planning on doing this, but decided to go for a 3-way setup with the 6.5 in the doorpanel, so don't know how it would actually work out.
My project is a 1991 BMW 318is (E30), if his car is an E36 body model (92+), then im guessing it will be very different. Let me know the exact year, to see if I can help you or not.
I don't want to go on a long rant if it isn't going to help you.