I can't be sure about our specific cluster, but I would generally advise against using
the main cluster harness.
A ton of electromagnetic compatibility engineering and testing is done to prevent the
cluster from becoming susceptible to environmental electromagnetic fields (such as high
power lines, airport radars, AM/FM radio transmitters, etc.) and/or from emitting RF that
could interfere with the operation of other onboard electronics.
Tapping into any main cluster lines, especially power/ground, could have adverse effects
on either the car itself or other electronics you have installed.
I would suggest using the clock wire harness or another portion of the electrical routing
in the car such as the cigarette lighter.
I know it might sound crazy, but trust me, I'm running electromagnetic compatibility tests
on the radiated RF emissions of GM gauge clusters right now.