I have an e36 with the factory head unit. It plays.... cassettes.
I do not have the CD changer in the trunk. I'm pretty sure I do have an Alpine CD changer from an earlier model e36 somewhere in the garage, but I'm not sure of compatibility and I really don't want to go in that direction anyway.
I'm also reluctant to put an aftermarket head unit in the car. I just don't like the way they look. I've never seen one that didn't look out-of-place.
I have an iPhone. A cursory search for iPod adapters turns up various options starting at around $150. That's $120 too much, in my opinion. I have been advised that the cheapies that plug into the cassette slot are not worth anything, and often don't work at all with auto-reverse units.
The ability to charge the phone and control the iPhone with the radio controls would be nice but are not important to me.
The ability to blast "Low Rider" as I pull up to Timing Slips after an autocross run, however....
"Low Rider drives a little slower"
I should add that my level of sophistication regarding electrical matters is only slightly better than cave dweller, so it needs to be simple/easy.
Thanks in advance for any help.

