The plug by the driver's side roll bar (LHD) is the rear window defroster for a hardtop or glass window soft top. There is no rear speaker wiring near the roll bar, you have to route the wires yourself from the roll bar to the back of the head unit.
Link to Instructions:
Porsche Boxster Rear Speaker Kit Installation - 986 / 987 (1997-08) - Pelican Parts Technical Article