Oh, BOY
Examine EVERY connector, headlights, bulkhead connector, heater, cluster, etc for damage due to heat, moisture, looseness, or corrosion.
IF you are using an original factory harness, I would untape the under --dash harness and examine the big "in harness splice." Easiest way to find it is to untape from the BLACK ammeter wire down. That will be the ONLY large gauge wire with a ring terminal.
Spend a LOT of time critically looking over the bulkhead connector and read the MAD article on the troubles they develop:
If you scroll down the page, you can see the simplified representation of the "welded splice"
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
which came from here, other articles:
http://www.madelectrical.com/electrical-tech.shtml
So far as the cluster, and depending on year, here is what ALL was wrong with my 67:
First, copper traces where the contacts of lamp sockets can get oxidized and corroded. Some discussion on an earlier thread suggested everything from vinegar and ketchup to soap and water, and scotchbrite pads.
In addition, the PC board(s) are "grounded" to the cluster housing with several mounting screws, and the cluster is "grounded" when installed with it's mounting screws.
THIS IS a poor system. Much better to install a ground pigtail wire to one of these ground screws, a foot long or whatever, and then bolt it to something like the column support.
The brass springy fingers that form the contacts for the instrument regulator MAY NOT be making contact with the board!! ON mine, I had to solder jumpers across from the fingers to the board
The "fake nuts" on the temp/ fuel gauges get loose / corroded.. After cleaning the board, replace them with real nuts, but don't go crazy tightening them
The harness connector pins get loose, corroded, break off. You can resolder them sometimes, or insert smaller wire inside the broken ones to shore them up. In my case, so many were broken, I gave up. I soldered small pigtails to the board traces at the pins --after scraping the coating off -- and used "Molex" style connectors I got from Radio Shack
Make sure you install a good (updated solid state) instrument regulator. You can test the gauges with known resistors which are the same for all gauges on these panels
(On a Ralley dash, the instrument regulator is built into the fuel gauge. I suggest you contact Redfish for the cure on those)
And of course don't forget the lamp sockets. Clean and bend the contact fingers for better contact, or if in really poor shape, find others.
