ABS wheel sensor

My garage told me that a wheel sensor has gone on my abs system as when the car is moving the abs warning light comes on, which it doesn't when it is first started and stood still. My problem is i don't know where the connecting harness's are for the front wheels, as i need to check the resistance of the wheel sensors. Plus do you check the resistance with the cable connected to the sensor or just the sensor. Or does anybody know what the diagnostic codes are and which pins to connect a voltmeter to on the diagnostic socket, to tell me the problem.
1990 GTi 2.0 4wd 4ws.