Exceptions to the warrant requirement.
The fourth amendment USC requires a warrant (issued by the court) unless one of the recognized exceptions to the warrant requirement can be applied. There are two possibilities here provided we have a key to the box: The Mobile conveyance doctrine allows an officer to search the entire vehicle and any closed containers therein if, there is probable cause to believe the vehicle contains evidence of a crime (can you smell the cannabis?). No arrest required on this one and the box can be forced open if necessary. Next is the Inventory Search following the impoundment of the vehicle usually after the arrest of the driver. This is a complete search, including closed containers that can be opened without breakage, provided the agency requires an inventory of impounded vehicles. We used to employ a third exception, Search incident to Arrest, but we have to change our tactics on that one in light of recent case law.
Set all this aside for a moment. If I really wanted to get into that locked cash box I would start by just asking the guy to open it for me. It's amazing how successful this can be.
Rule of thumb: Armed with Reasonable Suspicion of a crime, you almost never need a warrant to search a vehicle that is operable an in a public location.
Too many cops don't understand this and fail to take advantage of 5 different exceptions to the warrant requirement that apply to vehicles. Show me a bad guy who doesn't drive (legally or illegally) and I'll show you a real cop who actually eats donuts.
Life is a gift from God. What you do with it is your gift to God.
Concerned about Officer Safety, liability, citizen beefs? Use the best weapon you have. Be Nice. It's tough to attack, sue or complain about someone you like.