If you told me the problem came up immediately after the mechanic fixed the problem with your heater fan (resistor pack, yes?) then I would take it back to that shop and explain what happened. It's possible the tech accidently disturbed a ground, and somehow the taillight fog lamp and reverse lamps are completing an incomplete ground that serves the dash lights. The dash light circuit probably functions with only one ground, and that has been lost either through corrosion or the tech's mistake.

You could probably skip the troubleshooting by ferreting out the hot and ground sides of the dash lighting circuit where-ever it's accessible, and then simply install a new ground spliced into the old ground. That's what I'd try first.

If you check a manual you will probably find that the dash lights are grounded at the passenger footwell, behind the Fuel ECU. These are simple push-on connectors, and if you find one disconnected that may be your hint. Otherwise make sure the connectors are tight and clean if you do end up hunting around that area.

-Dylan