I think a bmw cd43 headunit will fit the e28 I think it does aux but not 100% sure

I also managed to dig out a nice original looking but modern pioneer headunit for a friend with an e36 it blends in very well with the dash il get the model no. For you

Lastly you could go the Becker route

It was actually clarion not pioneer

Here's a newer model


And a CD43