If this one has the seperate amp/tuner audio system, it sounds like the amp or tuner is not switching off consistently. This one can be a bear to solve but, I've found simply unpluggin the amp and turning the client loose with the car to be the most effective metheod of testing. I show them where the connectors are and intruct them on how to replug them in as well.
If thats the problem its either time to go aftermarket (which sounds better in these earlier cars) or remove the head unit, amp or tuner (which ever one is staying on) and send them the Becker USA for repair.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.