Wiring Questions

[Probe Dude]

As long as you can isolate both leads from the backlight from ground you can
have them dim with the oem setup (as Mark has shown).

A lot of gauges use the metal body of the gauge as the return ground path,
expecting the end user to ground the gauge body.  With prevalence of plastic
dashes in today's cars it is pretty easy to instead tie the gauge body to
the dimmer ground.  You do run into problems if your gauge is electronic and
is also using the case as ground or internally grounding the case through a
hookup wire.

A PNP transistor and some resistors and one could easily convert the factory
setup to vary the positive side to dim those gauges that cannot be ground
isolated.