If the cop has to ask, then don't give him permission.
Cops play on you to let them search you and your car, but will nail you if they find anything. Call their bluff, don't give them permission or hand the drugs over..
If he then searches without permission, then you get an attorney and the charges will be thrown out on illegal search and seizure.
If he has probable cause, then he doesn't have to search...
However cops will make you "believe" that they have probable cause when they don't. If they ask permission, say "No". If they keep asking, tell them, "No means no". They will try to keep pestering/asking you until they get you to say, "yes". All they need is one yes to a million of your "no's"... If it only worked that way with girls....
Just say "no" to cops searching your car.
If they ask why, tell them that every cop that has searched a car has left it a mess. If they keep doing that, they will never live down the "pig" knickname. And they have no right to mess up my car...