You can refuse to open it, and in fact, you can refuse a vehicle search altogether. The officer must have probable cause to conduct a search without consent. However, if he does have probable cause, he can not only conduct the search, but will either force the case open on the spot if you refuse to open it for him, or seize it as evidence to be opened at the station. Ditto for the trunk. It is also important to note, that if you do give permission for a vehicle search, but you refuse to open the case for him, he need only to confiscate it and obtain a warrant to force it open. IANAL, but the above applies is most US jurisdictions. Your rights may vary slightly form state to state.