Accessories on with Remote Start - Where to Tap?

[tenscourts]

Forgive me if this is listed somewhere else but does anyone know where I could tap to get power when the vehicle is remotely started before the key is inserted? The radio and certain accessories are powered but I'm already running a fuse tap on the RAP fuse and that doesn't supply power during remote start. I don't actually want to power an accessory. I just need it for my signal wire to a relay that feeds off an auxiliary battery. Use case: Rear seat headrests don't have power until the key is inserted. Need the ability to power them on while loading my daughter into her car seat before I enter the front of the vehicle. Thanks for any feedback in advance.