It could be the brake light switch, I believe there are 2 off them in there, one operates the brake lights, the other, not sure of its function.
It could also be the stepped rings on the drive shafts, the ones that the abs sensors work with, they are either clogged up with mud n carp or just corroded away.
An ABS sensor knackered.
As said above, the HDC switch and / or its wiring.
Hope this helps you out a bit mate and you get it fixed soon.
Gaz