tell them you have a sick neice to get to and that you have to take a dump.
be polite, but brief.
If they ask you to get out of the car, comply but ask for a supervisor immediately.
'may i leave' is nice, so is 'get me a lawyer if not'.
usually if you're hard at work like they, you get no grief. if you're drunk and causing trouble, they're gonna wreck your day.