maybe?
more details would make this question more answerable.
There are 2 things that commonly go wrong with the overdrive:
-the relay's solder cracks or the relay dies - simply fix with a cheapo soldering iron.
-the solenoid dies and has to be replaced or byassed. there's a special kit from ipd, maybe from fcpgroton too.

abnormally, the switch in the gearshift will die, or there may be an issue with really old tranny fluid or somesuch gumming everything up. In my frankencar, the wire to the solenoid came loose and rubbed apart on the driveshaft.

So, does the light go on an off with the switch? does the light ever show? does the car just never shift? what?
If the light goes on and off with the switch, the relay is probably working, and the wiring probably intact, in which case I would say it's an intra-tranny problem.
If the light stays on, check the wiring. The FAQ says where the OD relay is. It's rectangular or square I think

good luck
-Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond