I believe that its still within your 4th Amendment right to refuse consent to a vehicle search even if its a Rental Car. But we all know how inconsistently some LEO's adhere to the laws or honoring constitutional rights. In the heat of the moment your vehicle could be unjustifiably searched. Would it EVENTUALLY get sorted out with a judge? Sure. But that is likely months or years down the line.

Short answer, don't drive like a donkey which would lead to getting pulled over in the first place.