Its always fun to drive it like its stolen. If you are considering going back you are voting with your money. You absolutely need to let the boss know but I wouldn't expect anything to change other than their lip service to you.
Personally I wouldn't give them the opportunity with one of the most expensive mechanical things I will ever own (automobiles in general).
EDIT: You have head "F--- me once, FU, f--- me twice, F me.