Regardless, the use of relays in horn and headlight circuits reduces the job of the switch to carrying a tiny fraction of the current it dealt with using "standard" factory wiring.
There are a couple of basic electrical formulas for power (heat). Let's take a moment to look at them.
(Sorry, techno-babble to follow).
Power (Watts) = Heat = volts x amps. With the voltage being a more or less constant, you can see that as you reduce the amps in a circuit, the heat generated is reduced. A relay does that by using the switch to trigger the relay (the switch no longer carries 5 or more amps, it carries about 100 mA (1/50th the current).
Power (heat) is also: Watts = amps x amps x ohms. Reducing the resistance in a circuit will help reduce heating... but not as quickly or significantly as reducing the amps (which is squared in the formula).
The resistance causing a switch to get hot and melt is INSIDE the switch at its contacts... not in the circuit wiring. The only way to protect the switch by reducing circuit resistance is to service the contacts inside the switch.. not the wiring in general. Fitting a relay is immediately easier than opening a switch to service its contacts. Fitting relays is also generally cheaper than replacing the switch when you take it apart and lose pieces or give up and have to fit a new one anyway.
IR camera? Not in my budget at this time. With continually dropping prices... maybe someday.
Edited by dklawson, 24 February 2016 - 01:07 PM.