There was a basic head unit which had a built in amp and powered the front and rear door speakers.
the only one I have fiddled with personally is the mid system, which only has a black amp in the rear left compartment, which looks like and I would say its a fair bet is the same as this one from a bmw BMW LEAR E46 325i AMP Amplifier 65128383871 | eBay
https://www.aulro.com/afvb/images/im...11/10/1001.jpg
The mid system to my knowledge only has one box of tricks in the back, and that's the above amp, which has no smarts, accepts 5v inputs and outputs to speakers in the doors and the center speaker. For all intensive purposes you could put in a standard amp, as there is 12volts, a power on line, and speaker lines, and 5v inputs, all you need, but you would either have to make up a few cross overs or find an amp with specific speaker outputs to match the low/mid/high range speakers. the aerial for the radio runs up to the front of the car and plugs directly into the head unit.
Once you start looking at the system with the nav screen there's lots of boxes of tricks in the back - I am not familiar with that setup.
I just had a look at RAVE and its a bit confusing, there may be the rear radio unit for the sat nav system, and it might be the same radio unit on the low line radio system. But on the mid system like mine, its incorporated into the head unit.
its easy enough to tell if your car has a mid system and is missing the rear amp, just grab the big multipin plug, look up the pin out in rave, measure the speaker pins and see if you get the correct ohm's values, measure the two 12v rails to see if ones always on and ones head unit switched, then check the inputs, easiest done with an analogue volt meter as like my car you will see the needle flapping about to the music.
