In many states, the courts have ruled that your expectation of privacy in your vehicle doesn't carry the same weight as your expectation of privacy in your home.

It boils down to the "driving is priviledge, not a right" thing.
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She was standin' alone over by the juke box, like she'd something to sell.
I said "baby, what's the goin' price?" She told me to go to hell.

Bon Scott - Shot Down in Flames