ANSWER
For the wiring of the new gauge faces, the logical question is 'when do I want the gauge illumination to be on?' The answer would be - anytime the lights are on. Turning the light switch to park will turn on the old gauge lighting, so this position should turn on the new gauges also.
The new gauge inverter should have two wires, a +12 positive line (should be red), and a ground (should be black). The ground can be connected at any ground, whether that be to the body of the car or by tapping another ground wire such as the ground wire for the radio. A good place to obtain ground is the metal plate underneath the steering column.
For +12v, you will need to tap into the light switch. The illumination wiring system can't be used, because it is dimmed. The correct wire to tap is the +12v wire that powers the exterior lights (parking lights, tail lights). On 1993-1995 this wire is brown. A 'tap' type connector can be used to get power from this wire without cutting it. The easiest way is to pull out the light switch (which will be out when you do the gauges anyway) and tap the brown wire right behind the switch. The brown wire also goes from the switch to the fuse panel, so the brown wire can also be tapped at fuse 13 or 15 (it goes through both fuses to power other accessories; fuse 13 goes to the dimmer for other interior gauge lighting, fuse 15 to the tail and parking lights.)
Years earlier than 1993 may have different wire colors, perhaps others can comment?