You can get simplified diagrams at "MyMopar" but they are not complete. In some cases, though they are a great help because they are sort of simplified:
http://www.mymopar.com/index.php?pid=31
The two you need:
http://www.mymopar.com/downloads/1973/73ValiantA.jpg
http://www.mymopar.com/downloads/1973/73ValiantB.jpg
and we have a thread here where several Chrysler service manuals are posted free for download:
http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download
The closest thing to yours on there is the 72 manual. It won't be exact, but should be of help:
[ame="http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf"]http://www.abodyjoe.com/pictures/Mis...Serv%20Man.pdf[/ame]
You may have several problems The instrument cluster is supplied power through the harness connector to pins which form a connector on the printed circuit board of the dash cluster
It is very common for these pins to corrode, come loose, or even break completely off. If you can solder, you may be able to clean and repair them. The ones on my 67 were so bad, I soldered pigtails onto the board copper traces, and brought them out to a different style connector, commonly referred to as a "Molex" style connector. Don't know where you are, but got mine at Radio Shack.
You can also buy replacement boards for some cars:
http://premiumdashdecals.com/index.htm
http://www.classicindustries.com/mopar/parts/dash-components/printed-circuits/?p=1&sb=0&r=15
It also may be possible that the dash lamps are not getting power at all. Does the heater/ radio indicator lamps work?
Check the inst fuse in the fuse panel, do you have power at that fuse?
