Mike... welcome to the forum. On those D's that had the antenna wire... it passed down form the tail, under small roller in an opening in the canopy... for th canopy to move back and forth... then to the back of the seat and down to the radio. Someone posted a really great pic of the roller some time back. If I can find it I will let you know.
EDIT... found it. Go to the link. about half way down
http://www.flightjournal.com/plane_profiles/p-51_mustang/p-51_mustang_details.asp