the grounds work down to one behind the cluster about a foot from the plug.
could run your own...
here have a look at mine you can see the grounds down to one with the cluster out
post 194
http://67-72chevytrucks.com/vboard/s...=421767&page=4
the ground may have been feeding threw the antenna too
horns can rot out,have you found the relay for it?
would usally be above/around the fusebox