There are dog/car specific harnesses - basically something like a padded full body harness that'll fit around your dog (and provide cushioning in the right places) and hook into a seatbelt.

I can't recommend anything specific, My late dog wasn't fond of it, and we just carried him - but a seatbelt is designed for humans, and might not cushion the 'right' places in a crash, nor be much use with a squirmy dog.