Going crazy with new Duster and over voltage issue.

The most common cause of overcharge/ over voltage in these girls is VOLTAGE DROP in the electrical harness, and or drop in the ground path from battery to VR

HOW to check:

With key in "run" but engine stopped, connect one multimeter probe to the "key" switch feed on the ballast resistor. If in doubt, measure both ends of the ballast to ground--you want the one with the highest reading. Now connect the other probe --stab it into the top of the battery PLUS post. You are hoping for a very very low reading, and if it is more than about .3V you have a drop problem and need to look for it

Check the ground: With engine warm, and battery charged as normal as you can get it, run the engine at fast idle. Make this test first with all accessories off, and again with heater, lights, etc powered on

Stab one probe of your meter into the top of the battery NEG post. Stab the remaining probe into the mounting flange of the VR. Be sure to stab through rust, paint, etc. You are again hoping for a very low reading, the lower the better, and zero is perfect. If not upgrade the grounding from engine block to firewall

If you do not have a service manual, wander over to MyMopar and download one--free. You may have to settle for Dodge vs Plymouth, etc