First open and check harness connectors for corrosion.  Unplated copper
with 20 years exposure to heat and moisture ...etc.  While you're there,
write down all the nameplate data on the ABS unit.

The usual (4-5mph) test sequence checks wheel sensors, then positions
the valves in bypass (so pump pressure doesn't go to the calipers),
checks valve positions, then runs a pressure check on the pump.  Failure
of any of these things in the sequence can flag out ABS and turn the
light on.

Best bet is to ask around to find a brake guy with a test set for your
ABS.  My '89 C4 coupe has a Bosch ABS unit and our local Chrysler dealer
has a test unit that works. (Local Chevy dealer gave up on Bosch ABS
years ago, farms out the job to an independent brake shop who sends it
to the Chrysler dealer.  Chevy Service was clueless about this.)  Good
news is that several "no-loads" were making a couple of bucks off the
job -- keeps the economy alive!