When you say "not the reostat", did you try removing it and jumpering the 2 pins together? In general, the dash lamps need full 12 V anyway. My dash is brighter since I changed the 2 bulbs for the light pipes to forward-shining LED's.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans