wire coming out the back of my cab?

[mwmeador38]

well i just pulled the bed off my 66 to paint the cab
and found a black wire coming through a hole in the back
of the cab. it looks like it might be original because it has
an old looking rubber grommet around it. any idea what
it's for? it's just sticking out about ten inches and goes
nowhere. it about midway between the bottom of the cab
and the rear window on the passenger side. it's a highly
optioned truck if that makes a difference.

anyone?