My batt was dead today on my 90 after the car seemed to run fine yesterday. Not even lights would be on in the car. Nothing in first place ignition and no start. Got a jump and started it up, took it down to have the batt tested. As they tested it in the car, they said something was draining the battery continually....something very small perhaps(unless its intermittent) and I can't figure out what it is. They just said it needs to be charged so I'm charging it, and I guess Ill have to disc the batt overnight when Im not driving until I sort the problem. I dont know how long this has been going on but I had this prob(the batt dying on me about a month and a half ago. I drove up somewhere, went in about 15 minutes, came out and the batt wouldnt start, had to jump it. Its either an intermittent problem Im thinking, or the drain is super small and eventually affecting it over time slowly?. They tested the alternator and it tested ok at about 13 and a half. I didnt see any lights or switches left on in the car save for the rear view mirror click light which has no bulb in it(it was switched on). I wondered if this could cause a small drain even if theres no bulb in there or light going on? Before I take it to any electrical shop that rapes my wallet for a diagnostic I'm trying to see if I can find out what it is with a voltmeter/multimeter I just bought. I don't know much about electrical, but what would be the best way to check? Can I put the voltmeter on the connected battery when the car's off and while reading the meter, start pullin fuses from the fuse compartment and see if the reading goes away?
I also had a prob with the right rear stereo speaker a while back in that I seem to remember it was drawing too much power somehow in the wiring and crapped out because of it when I went to have it diagnosed. I replaced the rear speakers and had new ones put in shortly after that but its been a while. Is it possible a rear speaker could be drawing power and leading to the problem? They said a common issue was stereo amps drawing power when the car's off, but I wouldn't think the speaker itself or wire to it could be drawing power since it should be cut off at the main source? TiA.

Someone also asked if I have an alarm in response to my problem and I said I have a Vats bypass kit on it with the three place switch. Is it even possible the bypass switch could be doing it being switched to "on" to bypass it?