It's a pain in the ass but can be done. The passenger side is the easy side to start with.
First off simply get a deep socket of the right size and unscrew the switch. You will be able to pull out a little bit of the wires.
Next you will want to remove the kick panel on the passenger side One screw and one push in body panel plu. Be carful they break easy, especially if they have not been removed before. Now you have somewhat access to the wires and hole.
Get something that you can fish through the hole starting at the switch and working your way inside the truck. The best thing to use is a piece of balling wire or what I have is the wire that is used to support suspended ceilings.
Simply fish it through the hole attach the hookup wire to it pull it though and there you go.
Make your connection to the switch. I recommend stripping a little insulation off, solder, and tape.
Screw the switch back in and your done.!!!
If you notice there is most of the wires for the doors, speaker's, ect in the kick panel so you can also pull the switch and try to find the same wire colors in the kick panel.
Bryan