107 (SL) Rear speaker placement

I am assisting a friend who is the new owner of an '85 280 SL (Euro model). First, I have to say that, so far, I'm very impressed with the sturdiness and quality of almost everything on the car. I have little Benz experience, but tons of BMW experience, and the Benz is a far nicer product, from a quality standpoint, than any contemporary BMW. Now on to my question: the car has two small speakers in the lower dash, but no rear speakers. Did 107 cars ever come from the factory with rear speakers? If not, are there any locations in the rear area where speakers can be installed without clearance problems? (This particular car has a small factory rear seat in the rear shelf area, which the owner wants to keep, if possible.) Any help would be much appreciated!
Colin