Except you can most definitely do that! When I was a youngster, I went on a road trip with my high school friends. We made some mix tapes (dating myself a bit...), but the 87 Olds cutlass ciera brougham that we bought for $200 didn't have a tape player. When we passed a junkyard, we pulled the deck out and went inside. I told the guy that it was broken and asked if we could go look for one to replace it. My Dad's car had the version with a tape deck, so I knew what I was looking for. When we found one, we grabbed it and as we walked back through the office, I told the guy we didn't have any luck - it looked the same; he didn't notice the deck. It fit right in. Factory plugs and all.

More recently, my buddy put cruise control in his Toyota Matrix that way. Bought the factory parts and just put them in where the factory holes were.