Switch on when power of

Hi, I'm New in electronic, and I need a Circuit that Can monitor a negative 12V input, and switch on a Ralay when there is no input.

What I need is when a ground cable is disconected (input) , switch on a relay.. as soon as the ground is back switch off the relay.

Any Ideas. ?
Thanks In advance..

Something like this. If the SW is closed (ground connected), the relay is off. If the SW is open, the PNP transistor pulls in the relay coil. The parasitic power drain with ground connected is ~120uA plus whatever your existing loads draw. When the ground is disconnected, the circuit draws ~100mA (due to the relay coil) from the battery.