phathaway: Yeah, you shouldn't need that thing from cruthcfield. There's no way for me to be able to tell you for sure (depends on your specific brand head unit), so you should just try it and see.
Basically the the oem head unit just works as a preamp. It takes whatever source (AM, FM, CD, etc.) breaks it up into four chanels, and send it to the oem amp to be amplified and sent out the 8 or 10 channels. SO when you replace the deck with an aftermarket unit, it does precisely the same thing.
The only thing you have to worry about, is that the voltage on the aftermarket deck's speaker outputs is similar to that of the oem amp. If it's not, then you will need that thingy-ma-bob that was talked about from crutchfield.
wulley: does that answer your question too?