I was putting in a new tac did not have one before and after plugging it into the only plug that wasn't used I started the engine to test it. No tac and the light was on. so I went to the shop manual I had down loaded before and I printed up a wiring dia. Now I found the wires were in the wrong slots. So I changed then around and tested again success 
so If you are having problems check and make sure the guy before you didn't screw it up 
be for warned Jeff 