It's been a while since I've seen the stock sub as I pulled it a while ago. If you suspect the speaker is "blown" all you need to do is take a C cell battery and touch the positive end to one speaker terminal and the negative to another. If you match polarity the speaker will move outward. If you reverse the polarity it will move inward. It should make a decent thud noise. If your sub does this the voice coil is not blown. Personally I'd suspect the amplifier unit. You can pull the two pin connector behind the amplifier and measure 12V there. Can't remember if its always hot so probably best to turn on the radio when testing. Nice thing is the pins are inside the connector and shouldn't short on the body of the car if you do turn on the radio with it unplugged. You can also go grab an amp bypass harness for back there, install it to jumper past the amp and see if you get any sound at all...