68 a-body left indicator pulses

What is the charge gauge doing when this happens? Also try disconnecting the voltage regulator and see if it stops.
Here are 2 links to your wiring schematics that may help with troubleshooting.
http://www.mymopar.com/downloads/1968/68DartA.jpg

http://www.mymopar.com/downloads/1968/68DartB.jpg