What's that wire?

Whereabouts is this?

 

from the picture under the seat it looks like.

 

Probably a shield wire.

 

That is a coaxial cable. The bare wire is a 'shield' wire to keep other electronic noise and signals from effecting the signal that is on the center wire. Most likely (99.999%), the bare wire is simply a ground wire. Some equipment has different signal ground and power ground. But, unsure if anything in a truck is that complicated (I was an electronics technician for 15 years. I do know of what I speak).
Anyway, the bare wire isn't anything to worry or fret about. Did you have any specific concerns with it?
PS. If you are looking for a ground connection, for like CB power or something similar. I wouldn't use that wire. Leave it alone. It isn't meant to carry any current, although it is capable of such. You may risk inducing an unwanted signal onto the signal wire (the other wire coming from the center) if you tried using this ground wire on something that carried any current.
All this said, I do not know the specific purpose of this specific cable (both signal wire and shield). I only know from an electronics standpoint what a coaxial cable is about, why they are used and what should and shouldn't normally be done with them.