Help with mounting bracket for TV in 2600RB bedroom

[more tired since retired]

I'm getting ready to install a TV mount in the bedroom of our 2600RB. The mount area is already tagged by GD. My understanding is the block is metal and I can verify the location with a magnet. I've been told to place a 1/4" piece of plywood behind the mount and then screw it in. Is that a good way to go and what length screws should be used? I don't want to penetrate the outer skin (or ever come close). The TV weights about 9 lbs and the bracket about 4 lbs.

And when mounting the bracket, should I use wood or metal screws if I'm drilling into a metal backing plate and the interior wall?

I'll be installing a 12DV HD television and will be pulling wire from the wiring for the overhead fan. Any suggestion for another location to pull from? The lights are on a dimmer, so that wouldn't be an available source.