Answer: If this is a typical "Rear Air" system you have to have the front air on so the AC Compressor is running to have rear air. Unless they have a positive refrigerant shutoff solenoid valve or a thermal expansion valve (TXV) on the front heater box AC evaporator coil, they can't run the back air only, because the refrigerant will waste a lot of energy trying to cool the front AC coils. Even with a TXV on the front evap core, you will waste some energy chilling down the front evaporator coil till the TXV senses it's over cold and throttles off the refrigerant flow. But it will work. You just need a separate wire to engage the AC Compressor clutch from the rear air controls. Yes - on the heater hose from the engine if you catch it before the Y splitter sending the hot water back to the rear heater. If you want these to function as emergency heater water cut-offs in case you spring a leak while on the road, you need to put positive shutoff ball valves in both the supply and return lines. Otherwise the system can and will still leak 'backwards' through the return hoses - it's all pressurized.