Voltage Problem

Fusible link on most is difficult to measure directly because the terminal at the multiconnector has a seal.

You can use voltage drop to look for resistance from the battery to the main splice without running the starter.
Hook up the meter to measure voltage drop.
Turn on the headlights
Read the current on the ammeter
Read the voltage drop on the multimeter.

The headlights & tail lights typically draw 12 to 15 amps. That's significantly more than the starting relay and ignition typically do. The ammeter readings will confirm that or show otherwise.
Lets say starting draws 5 amps through the wiring, and lights draw 15 amps.
That amplifies the effect of any resistance by 3.
Voltage drop with three times higher current will be three times larger.

Charge the battery when done testing.