I replaced my in-door lights of my old car with some blue LEDs a few years back. To test the setup, I used a variable AC-to-DC transformer I got at Walmart for $15. It was one of those "universal power plugs" that had a switch to go anywhere from 1.5V to 12V at 500mA. I cut off the universal plugs and put on some 1A-rated insulated gator clips. To help with troubleshooting, you may start with something like this (you'll at least stop burning out LEDs right now), but just be careful that the clips DO NOT TOUCH when it's plugged in.
I didn't understand the results of my testing -- since the door light's source was 9VDC I needed to be able to use 9V. I accomplished this with a small-value resistor (100-300ohms, trial and error to find the right one) and no less than 6 LEDs in series. Any less and I'd get a nice POP POP POP out of the first LED within seconds. At most I could use 8 LEDs before all of the LEDs became too dim to see, and at no point could I used LEDs in parallel with each other. I had my 6 LED strip setup in my car for 2 years before I sold it, and none ever burned out.
I'm not sure about your AC voltmeter question. I hope this helps a bit. It was fun to turn out the lights and watch a single LED blow up when I plugged in the power at 12V
