ABS Light on 1987 190DT Always Lit

Greetings all!

I have a 1987 190 turbo diesel that has a problem. The ABS light came on a few weeks ago and does not go off. I have tried replacing the speed sensors and even the overload protection relay (you know, the one with the fuses in the top), but to no avail. I am afraid that the ABS control unit is bad, but do not want to make the ~$1200 investment to check that possibility. I also noticed that about the same time, the glow plug light quit illuminating when I turned the ignition switch prior to starting the car. It does however come on after the car is started and burns for a bit prior to extinguishing itself. Can anyone clue me in to what the problem(s) might be? Any advice would be greatly appreciated. Thanks!

John Q