I found my red w/gray stripe on my 2015 MS P85D was the only hot wire with my car sleeping, but it is not a direct connection to the aux battery. Unfortunately I removed the ground post retaining nut which lossened and lifted the ground lead to both the parking controller and the lift gate controller 2 bad things happened: 1) the lift gate would beep when triggered but not activate the motor (a car power cycle fixed that) and. 2) the parking brakes got stuck engage. Power cycling did no fix that and Tesla tech on the phone stated after a brief look at the logos, he thought the CAN bus was signalling the parking brakes to stay on. Strangely, the instrument cluster displays an error that the parking brakes are stuck off. In retrospect, I should have used PoisTaps on both the so called constant hot and the ground lead instead of unplugging any connectors and lifting the ground legs which I am certain caused the problem. Car was towed off my driveway in wheel dolly’s back to the SC.
While waiting for the tow truck I tested a pure sine wave inverter and found the 14-16 gauge wire (red w/gray stripe) had just enough current to get 100 watts load (good enough to charge my Boosted Board) but not enough to attempt to drive a 360 watt load to charge my eBike. At 130 watts the voltage dropped below 12VDC shutting down the inverter.
This means I still need to locate a firewall penetration to get a heavy gauge cable from the aux battery to the rear cabin.