As a police officer, I have dealt with this many times. There is a reason why I want to search you car, and it's usually along the lines of "I sense something's off, but don't have proper PC to search your car without your permission."
There are two responses to the request: "Yeah, I have nothing to hide." and "No, I do not consent to a search."
If you choose the "no" option, be prepared for some raised eyebrows, but refusing a search is not PC by itself.
You just say "no sir, I do not consent to a search." And then shut the heck up. The only other words out of your mouth should be "I'm frightened of you and would be more comfortable with my attorney present."
Follow all commands, admit you're scared of the cops, and refuse to discuss anything. The police will treat you extra nice because nobody wants people afraid of them; it is demoralizing to be feared.