Another new link showed up over at MyMopar, a '68 Plymouth manual:
NOTICE this manual has some of the Barracuda specific material for the Rally Dash!!!!!
Page 295 has considerable info on instrument panels including Barracuda
page 339 is Barracuda Rally instrument panel wiring
http://www.mymopar.com/index.php?pid=109
Here
http://www.mymopar.com/downloads/servicemanuals/1968_Plymouth_Service_Manual.zip