I may be wrong, but looking at the Painless kit, it looks to me like the solenoid must be continuously energized the whole time it's on......and those solenoids suck a ton of power. The inReserve just uses a momentary pulse of power to engage and disengage the switch. You don't need continuous power to keep the solenoid coil engaged.
If the Painless system does need constant power to the coil, then it won't work with your ISIS system to automatically disconnect the battery when the voltage gets low. I've put the inReserve on several GTM's here, and as long as you have your car wired correctly, it works like a charm. As I've said before, I would recommend manually tripping the solenoid when you leave the car for extended periods (more than 6 hours) to avoid deep-cycling your battery, but at least you have the ability for ISIS to disconnect it automatically if you forget. I've left GTM's sit for over 4 months with the inReserve installed and no battery tender and the battery still had 12.6 volts....the only thing you want connected full time to the battery is the stereo memory and the hot wire to the engine ECU, which is usually hooked up thru the starter cable power anyway, so no special wiring needed there.....you still hook the starter cable direct to the battery....not thru inReserve.
