I don't have a '54 GMC wiring diagram, but my '52 GMC diagram doesn't show a pink wire at the switch. Original wiring was cloth wrapped, so I doubt your "melted" pink wire is original. Either it is undersized or something is causing too much current to flow. Original ('52) wiring had a 14 gauge wire from the ignition switch to a junction connecting voltage regulator "batt" (10 ga wire), headlight switch (10 ga) and ammeter (10 ga). NONE of those devices are fuse protected.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.