Ok so here's my idea to run backup 12V DC motor power, but I'm stuck on a part of the wiring.
120V AC Plug---> 120V AC to 12V DC Converter---> SPDT Relay (This is where I'm stuck. How would you wire a relay in the event that the 12V DC converter is not putting out any power so that power can then be drawn from a backup battery..I think you need a solenoid in there too but I'm not sure how)--->12DC Pump