Mark,
I'm not *certain* about the TR4, since my experience behind the
dash is with a 4A, but I suspect that there should be a black
wire with a ring terminal that goes to the bridge of each of
the small instruments, to provide ground. With the metal dash,
this might be different - the expectation might be that the
bridge will ground to the panel, which is then somehow grounded.
The big instruments end up being problems for a few reasons. Again,
they need a ground lead under one of the thumb screws. The lamp holders
don't have a ground lead, but make contact through the barbs
that hold them in the holes, and this interface may be corroded.
Finally, the plastic insert in the bulb holder tends to break
down over time, making poor contact with the bulb (either hot
or ground). New bulb holders are available, and sometimes
you can repair your existing one, depending on how it has failed.
Good luck. This is an ugly part of the electrical system. While
you're in there, you should probably replace all the bulbs -
it will certainly make a big difference in brightness!
chris
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