Pete you bought a car that is over a half century old. The wiring was inadequate when new. GM cut every wire to length with little give because copper costs money. Money they would rather keep for themselves. For the same reason they used wire that was too small of a gauge to carry the load the distance required but they figured no one would notice. Fire departments noticed but they didn't buy cars.
In your case the load you placed on your wiring blew the fuse because of corrosion. The fuse is bundled in the actual wiring as a fusible link. You can replace it but the next time you touch that dimmer switch I will bet it blows again; because the switch has shorted out. So I would replace the dimmer switch first, then replace the fusible link.
You need to determine the amperage rating of the fusible link before you go to your corner Chinese import auto parts store (refer to your service manual for rating and were to find it).
Personally I do not like fuses. I prefer to use a circuit breaker that will keep your car from burning to the ground but not leave you stranded on the side of the road needing a tow home.
They come in amperage ratings of 5 to 75 AMPs (once again you ptotect the circuit by determining it's capacity (load and wire size/distance). The longer you run a wire the bigger it has to be. If you add circuit loads (such as electric fans or fuel pumps, etc.) you have to increase fuse size and wire size at the same time.