You can fit a 10" (EDIT - Shallow mount) behind the seat no problem. As others have stated, use a grille if you're facing the speakers forward so the cone and surround won't ever be in danger of making contact with the seat. I fit two 12" shallow mount subs in my standard cab, one behind each seat. The grilles touch the seat backs, and I had to give up one click of travel on the seat base adjustment (and remove the factory jack/storage box obviously), but it works fine.
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