Too many varibles. Could be speaker coil, a connection, broken wire or amp failure. If you have a digital VOM. Need to trouble shoot it. Hard to give an answer here... GL
Does your brake light on the roof lleak? If so, that could have damaged your sub, as the water tends to run down right on top of it. Thats what happened to mine, I was lucky enough to find a replacement fairly cheaply on Ebay.
The problem with mine was intermittent as well. It happened quite a while after I noticed and fixed the roof leak. Best of luck figuring it out! I know for me it drove me nuts when the sub would quit working!
I've got an '02 with an OEM behind the seat subwoofer and amp module that works perfectly fine. I don't use it anymore because I bought an aftermarket sub so it's literally taking up behind the seat space. The speaker might need replaced but the amp and box are fine. If you'd like to buy it off me shoot me an offer and I will definitely consider it. I know it's on a 15 amp fuse in the interior fusebox too, check that fuse which may be half-blown.