Please follow these instructions if you want fix your car.

measure the battery voltage with your meter.[12.8v at rest[no load] is a fully charged battery.]

using the battery ground as your ground measure the voltage at the starter relay post. Should be the same as the battery voltage - see schematic

measure the voltage at hot side of the fuse box. Should be the battery voltage but probably will be lower because of the voltage drop at bulkhead connector due to poor connections.

put the ign switch in the run position.
Measure the battery voltage and then measure the voltage on the battery side of the ballast resistor. Should be same as the battery voltage.

there you have it - if the voltages don't measure up you have found the problem. Remember this is a40+ yr old car so the ign switch may be defective.