Switch current flow from A to B based on C's status
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Rewire the switch as shown.

I assume that there is some reason you don't want to do this but you haven't explained it in your question.









simulate this circuit

Figure 2. (a) The simplest - unlikely to go wrong - solution. (b) A simple, no-component solution.

This is really hard to beat. Automotive relays come in insulated enclosures, take standard crimp terminals and are very robust.