ok I was finally able to duplicate exactly what you guys are describing!
There are 4 wires going into the dimmer switch. The yellow wire on top is the feed from the light switch and should measure around 12v when the lights are on! If you aren't getting the voltage on that wire with the lights on then you have a wiring issue somewhere between the light switch and the dimmer switch. If the voltage is good there the next wire to check is the Green wire on the bottom of the dimmer. This is the feed for all the dash lights. The voltage at this wire will be somewhere between 6 - 12 volts depending on where the dimmer wheel is in it's range with the light switch on. The wirring connector must be connected to the dimmer wheel to get voltage to the green wire! If you have voltage there then you have a wiring isssue after the dimmer wheel!! If that's the case you need to make sure the ground side of the lighting circuit is actually grounded!
Good luck guys I hope this helps!!
If you need help with the voltmeter Google it