Check the fuses...
A couple of things I've run across on my 88.
The parking brake pedal sometimes doesn't come up far enough to turn off the light. Ive learned to lift it with my left foot that last couple of inches.
Check/clean the ABS connector(s). Mine is on the driver side frame rail in in front of the rear axle, follow the the wire from tone ring sensor on the diff. Get some electrical contact cleaner spray.
Somethings I think make a difference but I'm not sure:
Having the e-brake on and holding the brake seems to make the ABS computer happier, which makes me think that back brakes might need to be adjusted, it's on my list. Also if that is the case then an old worn out brake hose might be part of the problem. The rear brake hose was replaced yesterday and the light only came on briefly where it had been a 50/50 thing, but it's too soon to be sure.