A-B Switch

Not sure if this is the forum to ask this in, but with all the odd situational configurations in an alarm system, this was where I thought I would find what I'm looking for...... Anyone ever see an A-B switch? Not an on/off, but a "power source A" vs "power source B" type of thing? My cooling fans and their temp switch are wired to an ignition powered relay, so when I shut the key off....so goes the fans. I want to wire the fan relay either to a timer relay (which have histories of failure) OR to the battery so that the temp switch has final say. I'd like to switch OFF this feature (back to the original ignition configuration) in the winter. If the relay fails and the fans come on at an odd time or run too long, my battery (redtop Optima) will be more forgiving in the summer than winter. As always, thanks in advance. Subtlety is EVERYTHING.

All I can say is duhhhh. It hit me how obvious (and stupid of a question it was) the second I read your response. Thanks very much....now I have to go - the short bus is waiting.Subtlety is EVERYTHING.