I have a 88 420.. and the gas cap gasket is / was the culprit. The gaskets don't seem to last too long because everytime you put gas in it twists and wears. Because of the pressure generated, when you stop it 'leaks' through the gasket and the smell gets into the trunk and then into the passenger cabin. I just buy a new gas cap from a local parts store for a couple of bucks and problem solved. I keep one in the glove box, so when the gasket goes, I just replace it .