The trunk release & fuel door release switches in the drivers side door panel are in reality a combined switch module. It gets a common hot (+12v) feed from a cabin fuse that also feeds the vanity mirror lights, interior lights, courtesy lights, trunk light - among others. So, if these lights are working, it is not a fuse problem.
This combination switch has three wires going to it, and they connect to it with a single connector. The common +12v hot feed from the fuse is a blue wire with a yellow stripe. The wire feeding +12v from the switch back to the trunk release solenoid (when the trunk release button is pushed in) is brown with a white stripe. (This wire goes back through the switch in the glove box, which disables it if desired). The wire that feeds +12v from the switch back to the gas door solenoid (when the gas door release button is pushed in) is brown with a red stripe.
To test, remove the door inner trim panel. (If you need directions, let me know). The switch will have a three-wire connector going to it. Unplug it, and test the blue wire with a yellow stripe (common +12v feed) using a voltmeter or test light to be sure it is hot. If it is not hot, the problem is in the wiring going from the door to the cabin fuse box in the driver's kick panel. If everything else in the door that is electric is working, it is unlikely there is a problem with the harness.
If it is hot, carefully jumper between the hot feed wire and each of the other wires (one at a time). The gas door and the trunk should pop open when each's respective wire is fed +12v from the common feed wire.
The most likely scenario is that you will find the switch disconnected from its wires when you get the trim panel off.
NOTE: The wire colors are from the wiring diagram for a 1996 Millenia. The colors may change (but probably not) for later Millenias (up to 2002), but the switch itself and the concept of a common +12v that is passed through each of two switches built into a common module will be the same. My 2001 Millenia has the exact switch my 1996 Millenia had.