I've had some weird problems with the coolant level warning light on my '87 
too. Try checking the connection to the sensor on the main coolant 
reservoir (not the one behind the license plate, the other one). The light 
shouldn't be coming on if the main reservoir is full. Make sure there's no 
air pocket in the top of the main reservoir.

The problem with my light was that it would not go off after starting the 
engine. After fooling around with it a bunch, I learned that if I turn the 
key to the on position for about 5 seconds before starting the engine 
(listen for the fuel pump to kick on and then off) and then start the 
engine, the light goes out. If I don't go through this sequence, the light 
will stay on for a while, sometimes a long while.

-Mark McCulley