The ground might just be the exterior of the sockets touching the frame or body somewhere.
what do the sockets themselves look like?
If they have 1 wire coming out of them, I'd bet dollars to doughnuts that that's what you've got.
If there are two wires coming out of the sockets, pull the bulbs and look in the bottom. If there is just one metal contact in the bottom, then the other wire is a ground. If there are two, you've got a double-poled socket that grounds through the exterior.
Hopefully, this helps.
