But I do get 12v when I connect one of the meter leads to chassis ground. Is it correct to conclude from this that there is a short in the ground wire leading to the relay.
Specifically, this issue involves a tail light relay that controls tail, marker, and dash lights.
What's happening here depends on the make, model, and year of your car.
You want to know what's controlling that relay and how.
Ideally, you'll look at the wiring diagram.
So, for the sake of discussion, let's take a newer Toyota as an example and look at the tail light relay circuit.
Here, the Ground to energize the Tail light relay coil is at relay Pin Socket 85 and comes from the Body Control Module from Pin 14.
In this case, we see that there should be Ground at relay socket 85 WHEN the light switch is switched on. (there'll be no Ground provided when the light switch isn't turned on).
So in this case, if you weren't seeing a Ground at relay socket 85 (lights on), you'd want to "back pin" the wire at Pin 14 at the BCM's connector and see if it's providing the Ground there.
No. It means you have an open circuit in the relay coil ground.
If you connect a jumper wire from ground to 85, then the relay should energize.
