i thought of using a SPTT rocker switch with two relays (one SPST and a SPDT) to use for an "ON/AUTO/OFF" switch up in the dash.
the top position would energize the first relay. 86 leads to the second relay, 87 is the acc line, and 87a is the battery line. so de-energized it would allow power through to the ACC signal on an M2 or similar as normal, but energized it would pull from the battery simulating a running car
the middle position would leave both relays de-energized, acting as a normal system.
the bottom position would energize the second relay that would merely interupt the ACC line, simulating an off car. lead 86 would be from the first relay and lead 87 would lead to the ACC line on the PSU.
this set up also gives me an easy way to reset the computer as well.