MG Car Repair: MGB battery draw, fuse box, battery cables

Question
I have a 75 MGB with a 1yr old battery. Recently it has started draining down to the point where the car won't start. A mechanic noted that my heater is on all the time and won't switch off, but I don't see how that could drain the battery while the car is off. Needs fixing though. I have a voltmeter - my question is how do I use that to check for battery draw.. and is there some sort of procedure for locating the culprit..?
thanks

Answer
Hi Aaron,
I quick and simple test to see if there is a battery drain is to just remove one of the battery cables and put a 12v test light in between the cable and the post of the battery with everything turned off in the car. If it lights the light, there is a drain.
Then you have to start unplugging things. The first is the wires on the alternator, then go to the fuse box and start removing fuses one at a time and when the test light goes out you have at least narrowed the search. Then look at your wiring diagram to see what is powered by that circuit. Unless it was the alternator, in which case it is a bad diode pack in the alternator.
Howard