I would expect that the amplifier remote on wire on the radio (blue usually) is not conected to the car's amplifier remote on, (usually a little pink wire on a '88 535i- so check which it is on yours before wrecking anything).
Mine is this type, with the antenna integrated into the rear window. It works fine with every head unit I've put in it. On these cars I bet that is used to turn on the the antenna amplifier as well.
If the antenna amp is not activated I can't see you getting very good reception.
Nick