I just put in a stereo in my 71 and it already had wires hooked up from the last one which was missing when I bought the truck.

So I hook the new one up using the existing wires, everything works fine.

Today my battery is dead...and I mean DEAD!..nothing.. not even a click.

I'm sure it has to be the stereo, and the stereo has a yellow wire for constant voltage for the clock or something and I think that may be the problem but I am wondering how to go about checking for sure and narrowing it down. I have a multimeter, but I'm not too good at using it.

Thanks...

Set the meter to the amp scale. Hook the meter in line between the negative battery cable and the battery terminal. If the reading is zero, move the meter scale to mA. An acceptable reading is 50mA. If you get a reading on the amp scale, start pulling fuses one at a time until the reading drops. Hope that helps!
Bob