No Current in Car

Check the quality of the connection where the negative battery cable attaches to the frame. Corrosion will give the exact symptom you're describing.

I concur, check/clean the battery cable connections at both ends, and also check/wiggle the main cable connector(s) on the firewall where the cables connect into the body... not sure exactly where that is on a C3, on a C2 there are 2 large engine-side cables that plug into receptacles near where the steering column goes through the firewall. If the negative battery cable is attached to the frame, make sure the engine has an intact ground cable to the frame also.

Been awhile, but as I recall the battery on the C3 of that vintage is behind the seat. So, a cable problem can be magnified. But, given the age of the wiring, it could be that something was broken during the replacement of the temp guage. Electrical problems can be very difficult to find -- I suppose I would start with a multi-meter to check voltage -- perhaps at the fuse panel to see if anything is coming through.

After you check all the points everyone else have given you, try this. Take a set of jumper cables from the battery to the starter and the engine block. Try again. If nothing, find the fusible links for the car. I believe on '69, they moved them down to the starter solenoid. In '68, they were on the voltage regulator on the driver side inner fender but '69 has an internally regulated alternator.

Use a test light or volt meter to check across the link for voltage. The connection look like a small Tootsie Roll in the wire. The fusible link is actually the wire from the Tootsie Roll to the connector, usually like 4 or

6 inches long. It is simply a smaller gauge wire, for example, if the main wire is #12, the link may be #16.