Edit: turns out there's an amplifier in the trunk, and each one of the six speakers has its own output. So that's how the Becker CDR-220, which has only four outputs, drives the six speaker system.
Original question: I have a 2001 996 with six speakers - two dash, two by the rear seats, two in the doors. But the CDR-220 head unit has only four audio outputs. So I guess the dash and door speakers run off the same output? Is there a crossover hidden somewhere in the wiring or do they all get the same full-spectrum signal?