I wired the (accessory) clock in my car with +12VDC on the 8A fuse labeled "4" in the engine compartment (it's the top 8A fuse in the wiring diagram, but my memory says the 25A fuse is on top making this 8A fuse the bottom one - but you probably know what I mean). This is the same source as the courtesy light above the glove compartment, and with your clock being in the instrument panel (mine is on the tunnel) you may find that the shortest wiring work will be to get the +12VDC from this light.

My clock also has an instrument light, that I connected with the rest of the instrument lights on the light switch (so that it comes on with the lights, plus so that it dims just like the other instrument lights).

A good place to connect ground (0VDC) in your case would probably be on the bracket behind the instrument panel that holds the ashtray (if it is possible to keep this bracket when you replace the ashtray with the clock), but any place on the chassis will of course do (as the entire chassis is grounded).

My clock is not exactly a precision instrument and has an error of about 2 minutes for every 24 hours, but atleast it ticks...

Brgds,

Fredrik