Often, bulb ground is lost because the socket is corroded, not failure at the main ground point. The way the sockets are made, the ground wire is only in contact by friction with the (rust prone steel) socket shell. I took all mine out and soldered the wires to the shell.

When looking for the main ground, there is only one wire harness in the trunk, follow it around and find where the black wires branch out.

 

RonJ, what do the numbers mean on the diagram and where can I find them?

Thanks in advance