Originally Posted by augy
I have no experience whatsoever with audio, but can you just hook up a multimeter and measure the resistance accross the sub?
No, you can't.
Impediance cannot be measured with a DC source like a multimeter.
Easiest way (like posted above) is to open up the thing and hopefully it's listed on the driver magnet.
BTW, 4 or more ohm impediance will work on all (4 ohm) car stereo equipment. 8 ohm just won't sound as loud.