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