I've fitted a VSR (Voltage Sensitive Relay) too, a great bit of kit, no mucking around behind the dash or anywhere else for alternator light wires etc.
Just the live from the Engine Battery, a live to the Aux Battery, and a little earth wire for it to all work.
Really is Fit and Forget. the only thing it doesn't do is emergency self jump starts. I wouldn't just stick a switch in to overide the VSR, as this will parallel both batteries, and if the Engine Battery is proper dead, paralleling it with the Aux Battery will kill that too.
Personally, if/when i end up in this position, i'll just swap the wires over....
HTH
Luke