Turn Signal Troubles

There have been some who pull a rear lamp socket to replace a bulb. They install a questionable bulb that has rolled around in the glove box for ages. So rather than put the socket back in the fixture, they lay the bulb socket in the trunk and start testing.
They get similar results. The ground path begins at those 2 tiny tangs that hold the bulbs base in the socket. They should shift the bulb enough to create a connection between the bulbs base and the socket interior wall. Then the socket exterior to fixture, and fixture to car body. A daisy chain of parts for a ground path. A break the chain anywhere is equal to a broken wire.
We all understand that you don't want the fault to be in the signal switch but... If you don't find the fault at a fixture, Manually operated switches are always suspect. The wiring harness is last on the list, unless of course there is anything aftermarket like a trailer harness involved.