Authorities Searching your Truck

was stopped by AL dot on side of road.He tried to open door but I had it locked.He said to open the door and I said do you have probable cause?He said lady havent you ever had a inspection?I said my company safety rating is 100,Ive more then my share and never been asked to open door.Then I told him the constitution states you must have probable cause to search a vehicle unless owner gives ok.He never did get in ,but I have saw KY dot all up in a truck