There are many places to pick up a dimmer signal, but a really easy place is the ashtray/cup holder light. Pull open the drawer, look underneath, and find the gray wire behind it running into the drawer. Tap into that. There are a bunch of other places too, shorter wire run (like the dimmer module itself), but harder to explain here if you aren't familiar with it.
For power, on a Caprice or Impala, cavity 19 in the fuse box should have an open unfused ignition slot in the lower cavity. Tin a 14-16 gauge wire and shove it in the lower cavity. Or get creative and modify your crimp terminal of choice to fit the cavity. Run it through a 5A inline fuse to the tach. On every '94-96 B-Body I own, I run this wire through a fuse to a terminal strip under the dash, and that's where I pick up any further low-power ignition accessories.
The next challenge will be where to pick up the tach signal itself. The PCM has a logic level output pin specifically for the tach (which does not work with vintage tachs unless you get creative, ask me how I know). A modern tach should work fine with a logic signal from the PCM. If I recall though, you will need to add a female receptacle to the PCM harness as it's a blank terminal for '94/'95. Or just pull the signal from the ign coil input. If you get the signal from the coil, you may need an inline resistor or else the tach may jump around. You may also be able to tap the ignition module as well, but I've never done it that way. I've only used the coil input signal or the PCM tach output.