Part of the problem may be the "trick." Probe the tan wire coming out of the headlight switch. This should be power for the instrument lights with the switch in "park" or "head" AND the dimmer twisted to the left for full bright
The trick is.....
THIS TAN wire then goes to the fuse panel, through the instrument lamp fuse, and then changes to orange where it feeds off to all panel lamps
Wiring diagrams "such as they are" over at "My Mopar"
http://www.mymopar.com/downloads/1964/64DartA.jpg
http://www.mymopar.com/downloads/1964/64DartB.jpg
From here:
http://www.mymopar.com/index.php?pid=18
http://www.mymopar.com/index.php?pid=26