Without any description of any symptoms, it's going to be kinda hard to diagnose. Assuming that the car isn't behaving like anything is wrong (since you would have mentioned it if it was, right?)
A) For that era stang, there may be nothing wrong with it- that's why they're called "idiot lights".
B) Something is wrong and it is popping a constant code, take it to a good technician and have them run the codes and tell you what's pooped.
C) Something is wrong and it is popping a constant code and you don't want to pay a professional to tell you nothing is wrong, so refer to jrichker's plan for running codes yourself using a paper clip. Sorry, don't have a link, but if you pm him, he'll give you the rundown- or somebody else may pop up with it. It should be a sticky by now, IMO...