mice under dash of 98 frontier how do I get them out of there. heat fan no longer working

You will have to take the blower motor out and vacuum the heater box inside. You may have to replace the motor.

Vehicle: 2008 Nissan Frontier w/V6 engine. Problem: Mice nesting on top of cabin air filters. Entry point: The cut out in the sheet metal around the windshield washer filler tube. The Fix: Removed filters and nest (which was the size of a soccer ball (no kidding)). Installed new filters. 'Plugged' cut out with sponge that I compressed and wedged around the filler tube to block the entry. This is temporary since the mice can chew thru it but its an external deterrent that tells me when they are about to gain access, at which point I replace the sponge with a new one. Haven't figured a way of permanently blocking this entry due to its inaccessible location. You'd think Nissan (which boasts innovative designs) would not have left a direct path for these critters to the air filter chamber, but I guess that's asking too much for a $30,000+ vehicle.