I don't have the full schematics - just the pages 2 through 5 that Krautrock posted in another thread.
My guess is yes they are grounded. Maybe the socket is in direct contact with metal housing?
I forgot where I got that schematic, I saved it to my computer. I see now, that little mymopar.com tag on the bottom though, just poked through that site and found it...
link to download the pdf...
http://www.mymopar.com/downloads/79_Dodge_Truck_Wiring_Diagram.zipedit:
link to the page with the schematics.
http://www.mymopar.com/index.php?pid=24
