a fusible link is basically a fancy and smaller mega fuse or a short length of wire that is placed in the circuit. the wire is usually a couple of gauges smaller than the rest of the circuit wiring so that section of wire "fails" before the rest of the circuit gets damaged. if it fails that means the wire melted and burned apart so there is no more wire left there just a nub of burnt wire that needs to be replaced for the circuit to be useful again. the other style of fusible link, similar to the mega fuse, used in more modern cars is like a clear plastic box, in the relay/fuse panel, that has a short piece of wire in it of a certain alloy that will melt apart in the same manner. the difference is the box style is easy to replace once you figure out why it melted in the first place. fusible links are meant to handle the quick start up loads of things like heater motors or the like, stuff that would blow a fuse because they would draw more than the fuse is rated for. the fusible link can handle the quick start up draw and then the motor draw becomes less once it is running so the rest of the circuit isn't overloaded. more money and they use a different type of holder. to each their own.
here are a few links to both styles.
use relays in anything over 20amps and you will have no problems. heater fan, headlights, rad fan, fog lights, wiper motor, fuel pump, horn, etc. if using LEd lights the draw doesn't really warrant it but then if you don't like LED after and want to change your circuits would be set up properly.
there is also the circuit breaker idea but these are sort of a slow burn, like a fusible link, and shouldn't be used in a circuit that is for electronic equipment. the breaker may take a sec to blow and by that time it is too late for the electronic part which is more sensitive to voltage. the nice part is that they will reset themselves. the bad thing is that they will reset themselves even if the circuit is bad. they will keep going until they burn out.
https://www.summitracing.com/int/par...54pt/overview/
https://www.aliexpress.com/item/FLK-...916592069.html
https://www.waytekwire.com/products/1367/Fuses/&pg=27
https://www.amazon.com/Bussmann-FLD-...79D6WZW1HA26J4
one here with a good explanation for relay wiring
https://www.12voltplanet.co.uk/relay-guide.html