Hi Ragrim,
I have single din Kenwood H/U In my VZ and in install it i used a plastic adapter, which has a pocket underneath. To solve the steering wheel control issue I installed a CAT 10 which controls all of the steering wheel's remote functions. There is no delay between pushing the steering wheel control and the H/U reacting, which I thought there might have been. The main reason I went for this method over a RCA conversion on the factory H/U was that I wanted to be able to play MP3 discs and I also wanted 3 lots of RCA outs. Another reason was that I wanted to be able to control my sub via my aftermarket H/U, and use the H/U's EQ's.
Hope this helps.
And by the way, if you want to keep your factory H/U but have low level outputs for an amp, and don't what I have decribed above, PHREDDY is the dude to see!