Trying to install a stereo

The short answer is, you need a GOOD wiring diagram for your car, and you really should buy either a reprint (paper) or a CD version of it. I mean Chrysler, not Haynes or Chilton, etc

But you can download a 66 manual on the site 'fer free, which might be close enough. The page numbers do not follow the original, so you'll have to play with 'em a little to find stuff

66 manual in this thread:

http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download

the link

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

Wiring index is page 289 in my browser

Valiant / Cuda stuff starts page 302