Just to expand on wire gauge and voltage drop. On my 65 Barracuda the book says my headlight wires are 18 Gauge. This is too small for the wattage draw in amperes. It should be about 14 gauge at 12.5 volts. And the voltage must go from the engine side of the firewall thru a Bulkhead connector inside and then thru the headlight switch, then back thru that bulkhead connector a second time. So I wire in a relay system and feed the headlights directly with a 14 gauge wire right from the lug on the starter relay. Bigger wire gauge and less connectors makes the headlights MUCH brighter.

Same thing on my son's 1974 D200 pickup. The book says the headlight wires are 18 gauge. But I cut them and stripped the plastic insulation off and did a measurement and found that the wire was really 20 gauge. Way too thin of gauge.

Wire is costly for a manufacture. Thicker gauge wire cost more. So they use the thinest to just get the job done. No reserve in power rating. So when I do a rewire job on a daily driver I will replace all 18 gauge wires with a 16 or 14 gauge. Any 14 gauge gets a 10 gauge if I can. All starter cables get upgraded to 2 gauge.
But wire also adds weight to the vehicle so this is why we have common chasses grounding. The better would be a true return ground wire for every item but this extra wire adds cost and weight.