The ABS control unit is by the glovebox believe it or not...
Open it up and take the glovebox shielding plates(can't think of a better way of describing them) out and expect to find the abs ECU. A dodgey plug connection is possibly causing you your tale of woe.
Had a feeling you'd say '88 - Doesn't help one way or the other! Around the end of '88 there was a change to the ABS system, when a small silver relay was deleted from the system.
If you have the relay, it is mounted to the right and above the ABS relay, clipped above a bit of metalwork, so not too easy to see. If this relay is present, it's the chief suspect.
Should have asked if the ABS light only comes on when the ignition switch is on?
not too sure if it comes on or not!
totally lost me on the relay but i will have a look tonight didnt get chance last .
ive had my glove box off lately so my have trapped or knocked a connection like jon said!
but will look tonight brian and thanks