10 speaker factory speakers/amp with an aftermarket HU? The only way to retain that stuff without major rewiring is to use a Metra 70-8591 factory amp integration harness. The sound is not the greatest but you can usually at least a better final result than factory anyway. When we deal with these older bimmers, we try to just gut the factory HU, bypass the amp, and install good coaxials in the rear with good two way components in the front. A decent 4 channel amp and call it a day.

BMW's unfortunately had a very over-engineered stereo from the factory.