It can sometimes be even easier especially in late model cars as you can purchase a stereo wiring harness for lots of makes which just plug into your original loom and either plug into new stereo or are easy to wire as they give you the wiring diagram.
Personally I have both Phillip's and Sony cds and stackers and best compromise seems to be indash CD/Tuner with stacker or if you really want to get fancy I just put a DVD system in the Pajero ( I know but I've got kids and it keeps em quiet when you do a 21hr drive)) and this plays through existing radio and gives me MP3's plus dvd's and still fits in standard din opening.