Electrical gremlin. Dart won't start. Desperately seeking insight!

Sounds like the bulk head connector gremlin has found your car.

The black plug that passes thru the firewall into the passenger compartment...

Yup. the huge big connector that passed all the wiring through the firewall to other circuits in the interior.

Start by reading this article for the hows and whys:

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

Usable but not always complete electrical diagrams from MyMopar:

http://www.mymopar.com/index.php?pid=31

http://www.mymopar.com/downloads/1972/72DartA.JPG

http://www.mymopar.com/downloads/1972/72DartB.JPG

And you sir, are ONE lucky SOB, because you have a 72, you can download a 72 Chrysler shop manual right here

The thread:

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

Der linky:

[ame]http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf[/ame]