Anybody having experience using the original Bose loadspeakers with build-in amplifiers together with an aftermarket radio.? I am struggling with bad audio, and cannot find out if I need to use Line out from the aftermarket unit or traditional audio output.

Well the vehicle was assembled with parts made around 1991. While the OEM parts may look good, I'd say they're well past their prime. If they were made of the typical materials, they generally don't hold up more than a decade after being put into service. Things like extreme hot or cold temperatures, humidity etc. can hasten their demise. If you live on the coast in say Los Angeles, you're gonna get smog and salt air that can eat away at the driver surrounds and the cones within a matter of years. If you live in an area with a lot of humidity like Florida in the summer, the same thing can happen...speaker rot.