like evryone else has said they can't enter your home without your consent or with a warrant same goes for motor vehicles so if a police pulls over your car and searches it make sure that it is clear you have not consented to the search and unless the officer can prove probbable cause to a judge all incriminating evidenve within said vehicle will not be useable against you.

that said it is my personal policy that whenever a police officer comes to my home or pulls me over and ask me to get out of the car I, in the case of houses I step onto the porch and close the door behind me (any evidence in plain sight may be used to incriminate you without a warrant) and in the case of car i lock my vehicle

NEVER let an officer inside your house unless they have a warrant or you called them there they will do their job well