If the bolts holding the car back seat in place are rusted, you can use a penetrating oil to help loosen them. Spray the oil on the bolts and let it sit for a few hours before attempting to remove them. If the bolts still won't budge, you can try using a bolt extractor tool or a breaker bar to apply more force. Be careful not to strip the bolts, as this can make them even harder to remove. In extreme cases, you may need to cut the bolts using a reciprocating saw or angle grinder.