Buy a riv-nut kit from eBay. The bolts alone won't help, you need the
plastic inserts in the holes the bolt goes into. If Best Buy is installing it,
they should have what's needed to bolt the Fender sub to the under side
of the deck, even if they have to do some minor fabricating. I also have
one of these for sale, with and AMP and an AMP add on adapter for stock
or other radios without RCA SUB outputs.
The BOLT heads are 10mm, in any case... But, you won't have the plastic
nutserts they go into, in the square holes in the deck.