Frank,
If you have the 8 pin DIN then look under the passenger seat to see if you have an amplifier under there. Assuming the amp is under there, yes, the speakers should be plugged into it. I guess we'd need to see the back of the replacement unit to see what adapter you'd need for the preamp for the 8 Pin DIN. If the head unit has an RCA output pair for the preamp you should be able to get an RCA to DIN adapter.
Here is the wiring diagram, per Bentley, for the preamp setup:
