looking for amp suggestions

Well definitely don't use wire nuts. Those are never advised for multi-strand applications.

As far as mounting it under the seat you can secure it straight to the floor... maybe using some wood blocks to space it up above the vent that you're worried about. I pop some silicone on the underside of the body if the screw happens to pop through the metal. As far as grounding it... the best way is straight to the chassy. I usually advise finding a nice spot of metal, sanding it back to bare metal, and popping a self-tapping screw through with a star washer attached.

I would not use wire nuts now, but back then it worked just fine.

I found a better place to mount the amp than under the seat. Apparently there is enough room behind the back seat even though it doesn't look like there is any room. I went and talked to the guy's at a local shop and that was the first place they mentioned. I also did more searching on the net and found some threads with pics and it looks like there is room for several amps behind the seat.

Now I just need to decide which amp to get, there are to many choices.