I've done some more reading on here and found the pelican wiring diagram for the brake light circuit,
http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part3-1.jpg.
So, what I'm looking for is the T2f connection as I suspect that these wires have been tampered with at some point and that whoever did it forgot to wire the switches to the brake lights (the bk/ye wire from 81 on the switches only run to the cruise control unit).
Does anyone know where that connection is actually at in the car (and what it looks like)?
Cheers, Björn