The back of the factory HU fits into a hole in a tab made of sheet metal. I bent that tab down, toward the back of the car, until it was horizontal, so the the back of my new HU rests on it. I needed to either do that or get rid of it anyway - there's not much room back there.
I put in an Alpine 9825 and used the preamp RCA outputs in the rear. These outputs interfered with the rear wall even with 90 degree elbows. The wall behind the decks is the air channel box for defrosting and upper vents in the dash. I actually used a dremel and carefully cut away a small rectangle so that the RCAs would go in this small hole and the HU would go far enough back. I used some aluminized tape (what you would use to tape up sealed liners like for a wood floor) to cover the opening but pushed it in for clearance for the RCA's, and then everything went fine. I was real disappointed I had to do this, but I used VRumVrum's faceplate and mount, and it looks great. Most people wouldn't even know its not stock.
I only write this so you can be careful on the HU you use, and consider if you are going to use pre amps out, how they physically hook and fit in the car.