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just my guess, but it sounds like there is high resistance that has developed in the circuit with the 18 gauge wire. with the high resistance with no load (just checking for voltage) you will show 12 volts. as soon as the load is applied (connecting to running lights) the voltage wil drop and lights won't work. using the wiring diagram start checking the connections from the batttery back. maybe even the simple act of disconnecting and reconnectiong a connection will clean it enough to make it work better. even remove fuse and check condition of connectors.

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Anyway, the problem seems to have been high resistance in the supply wire for those circuits to the fuse box due to corrosion/deterioration in the removable female spade terminal.

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