First off, sorry about the long title as I don't know a better brief of my situation.
The bus has it's own electrical systems that I don't want to take away, but i need to control the stop lights, etc from the semi truck when it is attached. The semi truck by design of course can power lights on a trailer so I will be using it in this way to control the lights on the bus.
I want to have a bypass switch. If the bus is powered on by the master disconnect switch, then I need it to lift the ground from the semi truck power feed...thus turning off the control of the lights from the truck battery
I'm thinking to tap the bus positive, post master disconnect switch, to feed relay point 86. 85 goes to the bus ground. This same ground is where 87a, via 30 allows the semi truck to complete the circuit to control the lights.
When the bus master disconnect switch is switched off (normal usual operation), the ground from the truck connects to the bus allowing the truck system to control the bus lights.
When the bus master disconnect is switched on (rare but to operate the bus systems independent from the truck), the ground from the truck should "lift". This will assure that both battery systems are not powering the same circuit.
Is this a good way to do it even though I'm only working with 12v on either side of the relay? Am I missing something?