Try something like the attached schematic. It's just an [inverting] buffer to drive either one relay or the other.
When the switch is pressed, the input to the LM555 becomes low, which causes the output to switch high - this causes current to flow from the output through RLY1 to ground.
When the switch is released, the input to the LM555 becomes high (due to the 10k pullup resistor), which causes the output to switch low - this causes current to flow into the output through RLY2 to from the 12V supply.
There is a capacitor shown across the switch, which may be optionally connected to reduce relay chatter caused by the switch contact bounce (though probably can ignore it due to the slow speed of the relays).