Subwoofer Intermittently Working

I noticed this over the last couple of days. For some reason my sub will not work some days and other days it works fine. I thought it was because of the temperature because it always worked when it is warmer out. My question is how do I go about troubleshooting this? Is the speaker, amp, or fuse causing it to intermittently work? If I have to replace the sub, am I better off getting a OE unit off eBay from salvage RDX or can I get a aftermarket sub to fit in the factory box?

EDIT: This is on a 07 non-tech RDX