When I've had a situation like Traveller, I've never hesitated to bring the keys back and request another car. It has resulted in some grimaces from the customer service people sometimes, but I always got a cleaner car or one in better shape, even if it meant I had to be given a free single/double/triple upgrade. I'm never rude, just politely firm about it.

Never hurts to ask, not necessarily for the upgrade, but for what you're paying for, a clean car with nothing more than minor scratches/wear and tear.

Just my $0.02..