Welcome to the forum!
What year is your Taurus? From your profile, it appears you have a 1997 Taurus, but it would be helpful to post that info when starting a thread.
When you measured 4 to 6 Vdc at the switches, was that using a ground at the switch, or a common ground from the body of the car? If at the switch, you may have a bad ground for the door or the switch. A bad ground could easily effect the intensity of the LED switch lighting as well. If using a body ground or similar, then you may have severe corrosion on one of the feed wires to the switches, likely where the wires go through the door jamb. There is a cylindrical connector between the body of the car and the door. I've seen wires get damaged or corroded fairly often in this connector. Not fund to repair, but I imagine it's not fun to drive with a window part-way down either.
If you measure the voltage to the accessory delay relay in the instrument panel fuse/relay box, do you have battery voltage there or only the same 4-6 Vdc?
-Rod