check behind the car before getting in
Look behind you and BEEP the horn.
taje
the car backing up
That would be the car backing up because you can't see the other car coming at you and more than likely the other car wouldn't see you either.
If it only happens when backing up, you may have the parking brake engaged.
it is the tranny mount
The car in motion is ALWAYS at fault when it hits a stationary vehicle.
The backing. The other car has the right of way wether they saw you or not doesn't change that. If it was parked, that's even worse.
Assuming that the car hit was stopped when the other car was backing up, the one backing will be at fault. If there is no damage, or if it is below a certain threshold it may not even count as a collision.
The driver backing up. Unless you can prove the other car was driving unsafly.
In most states, fault lies with the person who had the "last clear chance" to avoid the accident. If you saw the car backing up at you and just sat there and let it hit you, it's your fault. If you saw the car backing up and thought "I can get past them" and instead hit them, it's your fault.