Ok --the '71 duster is needing some help so here goes....

When you place the car in drive and let run the lights will be kinda on the
dim side. But if you rev the engine a little the lights will brighten up.
So here's what Im gonna throw out there....
*The voltage regulator is brand new and the alternator tested ok at the local autozone but could be something bad about my alternator it didn't
pickup(?)
*Maybe my idle is set to low to keep everything properly charged???
(not putting out enough at that rpm???)
*Bad ground or possibly needing battery cables replaced????
*A possible dead cell in the battery??
Think Im close with any of this??

This is an extremely common problem with all older Chrysler products. Your alt could be a little weak to boot. Check that the idle speed is to spec, I like to run the smallest alternator pulley possible (2.5", I think?) on my daily drivers. That speeds up the alternator relative to engine RPM. There are other solutions, but these are the most basic, and a good starting point.

D/W

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If it ain't broke, fix it!