88 Cutlass Ciera...3rd brake light won't work?

My car failed inspection, one of the reasons being the 3rd brake light (one on the rear seat) would not work. So I replaced the bulbs, using a 1156 (the old one had blown out). Now here's the problem:

IT WON'T WORK!

I've tried 3 different 1156s, 1157s, and it still won't light up!

My cousin who's a mechanic did a circuit test and it lit his tester up without any problems.

Does anyone know what might be wrong here?

Yes the rear tailights/signals work, it's just the 3rd brake light doesn't want to :(

It's probably the connector where the bulb sits and not making contact properly. The tester works because he is poking at the connector with a pointed tester. Try cleaning the connectors and use a di-electric grease on the connection. If it is barely making a connection and power is applied then it cause a spark on that connection and you got a no power problem where the connection between the bulb and connector is corroded by then.

Had fun with a multimeter...it's showing there is a complete circuit through the light. Perhaps it's a grounding issue, which is what it seems highly likely of being, and if so, how do I resolve this (without having to pay anyone to do it >_>)?

I don't get this.

I verified with a multimeter I got from radioshack that I have 12 volts going through the bulb, and I have at least 20 milliamps going through it as well.

I probed the side of the bulb (where the ground should be coming out) and there was a good 20 milliamps coming out of it, and there indeed is a closed circuit happening here!

I don't understand this???? I have an 1156 bulb in, that is the correct bulb is it not?