Stereo & Electronics - Serious electrical Problems

This LS1 had its alt taken off and tested at autozone. They said it was fine.
We put it back on the car (attached the 2 cords). Everything was fine for 15 minutes then all of a sudden the electrical gauge on the dash Completely dropped to 8v!!!

I've heard the gauge on the dash just shows how your alt is running. Is this true?

The heater was blowing very low and the whole dash was very dimmed.

Please help!! Where do we get started?
I have a voltmeter.

I don't know how much I would trust the Autozone techs, but there are a few things you can try before taking the plunge on a new alternator. Of course clean the connections, wire brush or sandpaper. Make sure the small gauge regulator wire is securely plugged into the alternator. If you can, have a helper watch your voltage while you wiggle that control wire around with the engine running, make sure you don't get wrapped up in the belt though:|. Also make sure that the stud (on the alternator) for the main power connection didn't loosen up on you.

yea, its neither of those
could anything be draining the battery? I'm thinking no..