when the headlights are on, the dash light(s) are on & likely there on the dash backside printed circuit it is bleeding over (improperly connected to) the ~5 V output from the plug in voltage limiter in the printed circuit & this puts full voltage to the gauges. Keep the lights off (drive in the daytime!) or pull that wire out of the headlight bakelite connector to disable that circuit (
www.mymopar.com will have the colors/schematics for you) till you solve it . I'd pull the dash printed circuit again & go over it with a fine tooth comb. Holler how it goes.