What I have done in the past for #6 - #8 finishing screws thru the carpet, is to go underneath first and poke a needle thru the carpet in the center of the hole. Then get a Prestolite torch and heat up the tip of a 16D nail (while holding the nail with a pair of pliers, then push the needle thru the carpet on the topside and melt the nylon carpeting with the nail-head.
But for larger holes, (I suspect you might need a 1/4" or 5/16" hole for the seat belt), you could use same procedure as above except use a tapered center-punch instead of a 16D nail.