sorry chris I just read darrens post and it made me reply a bit unfairly.I am at work now and looking at autodata manual.
If its bosch abs 5.0 the warning light circuit is earthed through abs unit on pin 40. This shows a blue and red wire. Bare in mind autodata can be wrong.
I would advise finding wire and using power probe see what you have.
If circuit ok you should be able to ground wire and abs light comes on. If not you have warning light circuit fault. Look out for disconnection somewhere most warning light circuit faults are due to disconnection.
If warning light circuit now appears ok fault at abs unit.
Check power supplies and earths.
power in on pins 4 , 2 , 3
earths 1 , 15
brake light on 28 live when pedal pressed normally and ecu needs to see bulbs when not ie resistance to earth.
forget sensors etc until you get warning light circuit working.
hope this is more helpfull
john
ps i will watch post and add more if you need it