TKG,
Please correct me if I'm wrong. I thought the switch is opto coupled to the FET that passes the voltage to the receiver. This means the switch leads cannot pickup RFI but the FET can still transmit ignition noise. So if the ignition battery is the same as the receiver battery there is no RFI protection from the ignition feeding noise into the receiver. The unit is not designed to work as an ignition battery eliminator.
John