This picture is from a 95 3-series, but will cover most all radios that have this particular plug, its the more common one in 88-up 5-7 series and 92-up 3 series. Please buy the adapter harness (and antenna adapter if needed) from a car audio shop or a Best Buy/Circuit City type place before cutting your harness. It'll make your install WAY more simple.
Metra part numbers.....Amp integration harness........Standard radio
3 series 92-01___________70-8591__________________70-8590
5 series 92-01___________70-8591__________________70-8590
M Coupe 98-00__________70-8591__________________70-8590
2002-up Antenna adapter: 40-EU10
Most pre-88 radios that look like this:
Unfortunately, no one that I am aware of makes a harness for these older radios. The do make speaker harness adapters, for the round-button plugs only, so you'll have to hack it up. See This post
for an install that I helped a guy with. Any other questions, feel free to ask...