I have a 15" Rockford fosgate p2 and I built a floor where the spare tire goes and you can barely hear the sub. I do not know what else to do to the thing. This is my first time building one so any help is needed!
I don't want to sound too critical, becuz we all have to start somewhere. But I think u should have done some more research before u built anything. I'm guessing that u saw another vehicle w/ a sub in the spare tire well that sounded good, and u decided u would do the same. But successfully installing a sub in the spare tire well is actually on the complicated side, and is generally not a good project for your 1st install.
That being said, Josh already hit on the biggest potential problems. From what it looks like in the pic, there are no ports in the holes in the baffle. That means the sound waves from the front and back of the sub are not separated, and therefore will cancel each other out. That results in very little bass output, no matter how much power u dump into it. Ports are absolutely critical for a vented enclosure, and they must be carefully matched to the sub and enclosure.
Also, u need to verify that u have proper airspace in the spare tire well. It will need to be reinforced as well, so that it doesn't flex. And the baffle needs a solid, airtight seal. Also consider a grill or some type of cover so the sub doesn't get damaged.