The Brown/Black (Br/Sw) wire is a ground wire (pin #7 on X400 --> X498 ground point in trunk).

The Brown/Black (Br/Sw) wire on the 6-pin (X18830) is also a ground wire (pin #6 on X18830 --> X493 ground point, also in the trunk).

The other two wires on X18830 are as you point out - Red/Black (Rt/Sw), which is fused +12v (from fuse #86) and Green/White (Gr/Ws) which is the start trigger to the aux. heater.

If you can measure an audio signal on this Brown/Black wire, then there is a ground problem somewhere.