2000 Ford Taurus head lights

It is normal for both to come on when you pull the switch back. That is know as the flash to pass setting on the headlights.

As far as not being able to see with the high beams on, are the headlights properly adjusted or do the bulbs just get dim?

I am going to try to explain the problem better. When the headlights are off and you pull back on the dimmer switch, the the the high beam element comes on. When you turn the headlights on and the dimmer switch is in the middle position the low beam element is on. When you pull back the dimmer switch into the flash to pass position both the high and low beam elements are on. When you push the dimmer switch ahead to the high beam position, the high beam element comes on , but the low beam element goes off . I think the low element should stay on when the dimmer switch is on high beams. Where could the wiring be changed to correct this problem. I believe the wiring has been tampered with , as I have the same problem on my 91 Ranger that never used to have a problem. Thank You

Based on your description, they are doing what they should. In the forward position, only the high beam should be on. The low doesn't stay on with it. The flash to pass function will turn both on.