Firewall hole in 4runner

up about 4" and about 4" to the left of where the brake pedal goes through the fire wall....if the better is on the drivers side,,,but i don't think it is,,,,so go through the passengers side fire wall, there may be a boot from the wires comming through for all the control moduels.  But im assuming your running 4 Guage for that mrd-m500, i would drill a hole specifically for it, and put a grommit in to protect the wire from getting cut.

Run your rca cables on the opposite side of the truck as the power wire.  I have seen a led get lit up just from radiating power from the power wire.