Crowd sourcing opinions are not going to help you.
Drinking is legal except for those told not to drink by their doctor, those on probation and those under age. Drinking at a PUB is legal.
Half of the opinions on here are telling you you can legally be drunk in the back seat or legally drunk in passenger seat.
It is dangerous to sleep in a car.
Michael Jordan's father was sleeping it off in a car. Michael Jordan's father is the most well known person murdered in his car. He was not the only one murdered.
If you are going to get too drunk to drive, do not go near your car.
All these posters telling you you can legally sleep drunk in the back seat may be right, but you that action may still get you arrested for a charge other than drunk driving like vagrancy.
Do not drink then sleep in a car.