I don’t think the Ford Explorer is old enough to have used a carburetor. I think exhaust fumes are getting into the cabin. Either there is an exhaust system leak which could be from the manifolds on back to the end of the car or the tailgate isn’t sealing (assuming you don’t drive with the tailgate or tailgate window open). As a car moves along, a partial vacuum is created at the rear end which can suck in exhaust fumes. This is not a safe condition and not a normal condition. You should never smell the exhaust emissions inside the car.