nagager
A nagager is someone who sits in the back of the car and yells at the driver like they're a rookie. They don't drive, but they think they know everything.
'You're driving like a drunk squirrel, nagager!'
'Nagager, I'm not even lost. You're the one who took the wrong exit.'
'Nagager, if you don't shut up, I'm gonna let the GPS talk to you.'