Dash light relay??

64 is right. The dash lights for about a thousand years in Mopars work like this:

Power for park/ tail/ and dash lights comes from the tail light fuse. This feeds the dash lamp dimmer control, the output of which is traditional TAN which goes down to a separate special little fuse in the fuse panel. The dimmer controlled voltage goes through that fuse, comes out on ORANGE, and is distributed to all the dimmer controlled lamps, radio, heater, cluster, etc.

You can download a complete 66 shop manual for free here:

http://www.mymopar.com/downloads/servicemanuals/1966_Plymouth_Service_Manual.zip

Simplified diagrams which are not particularly always correct or complete, but which CAN be handy here:

http://www.mymopar.com/downloads/1966/66ValiantA.jpg

http://www.mymopar.com/downloads/1966/66ValiantB.jpg

Look at diagram "A" find the light switch and the TAN wire coming off. This is the output of the dimmer control

Follow the tan off the page at bottom right and to diagram "B"

Pick up the tan going to the fuse at the far lower left on diagram "B"

Notice that at the other end of this fuse are several ORANGE wires, and in fact one of 'em goes right to the radio. This is the dimmer controlled indicator lamp in the radio.