its covered in the nissan service manual abs diag howto
it also covers basic diagnostics tips (even with consult) and what to check for
when was it working last (was it ever?)
i expect the main ABS unit which from memory could be part of the boot mounted computer
looks at wheel speed on each wheel, likely TPS and main vehicle speed i would assume
has anyone been hacking your loom? any inline hackers like SAFC? RSM? speed cutters etc?
these shouldnt affect it, but maybe someone has done a botched install and cut one of the ABS sensors
i would suggest
check wiring and main ABS control unit (under bonnet, left had passenger side at the front)
check all fuses and basic sensors from nissan consult (cable and software for sub $100 on the forums)
read the service manual for basic diagnostics
fialing that, goto nissan dealer and ask to fix but be prepared for $$$
if the light is on, your ABS is disabled
that is, if you are driving and lock up the brakes, ABS wont fire, and you will skid
when ABS is working the light is off, when you lock the brakes, the ABS unit fires a rapid pulse on the brake fluid line
this in turn allows the brake calipers to open/close rapidly, then is turn, removes the "skid" properties of the wheel and lets you gain control instead of lock and skid with no control