Welcome to Stovebolt.com, GarthMC,
The Z-series GMC truck (54/55) had a completely different dash and radio than in a 1954-1955-1st Chevrolet truck.
Do you still have the headliner?
Do you have a radio?
If yes, do you still have the speaker above the mirror location, in the headliner?
That lead might be a speaker lead. Can you find the other end of the lead, under the dash?
You might want to also join oldGMCtrucks.com for excellent advice on GMC trucks.
