How do I change out the driver side headlight assembly on 99 Dodge Durango?

Looks like Second generation (1997-2004)

https://en.wikipedia.org/wiki/Dodge_Dakota
Comparisons

https://www.google.com/search?q=2001+Dodge+Dakota+v+Dodge+Durango

you can also open up a picture of your year and compare to Durangos by various years.
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The circuit has a central timer module that is behind the driver side kick panel. The module would need to be tested with a meter.

you can change the valve inside of it its like the valve on a tire

Battery removal will not work. What will work is this: Use key to lock and then unlock driver's side door. I unclipped both horns at passenger side of engine compartment at front because they will go off while you are doing this procedure. I used this procedure last night after a rear passenger accidentally set of my alarm. I know this works! My vehicle is a 99' Durango.

Look at where the (3) lights come together, just above the lower light, and in the seam between the headlight and side marker. There is a small screw hidden in there. A "T-20" I believe. Remove that screw and persuade (gently but firmly) the side marker to come out.

You have to remove the headlamp assembly.Just remove the bolts holding the headlamp assembly to the fender and then just pull out.Now you can remove the bulb.Check and make sure it works before reinstalling it.Good luck.

Remove the door panel and find the window motor Unplug it and using a separate car battery run cables directly to the window motor When you switch the polarity the motor will either bring the window up or down or if noting happened change the window motor and window regulator assembly.

it has three 10mm bolts and one alignment pin.

Did you take a voltage reading at socket of dim one compared to bright one?

the headlight bulbs are accessed from under the hood behind the haeadlight assembly. they are removed by grabbing the plug on the back of the headlight assembly and pulling while turning counter clockwise. if you are replacingthe whole headlight assembly there are two 10mm bolts, one is under the headlight above the bumper, the other is behind the headlight and can be acessed by lifting the hood to move the grille and reach in next to the head light.. good luck hope i could help