Instead of trying to modify a truck harness, I would use a Hotrod Wires 12 circuit. You can custom route the wires wherever they need to be.

I used one of these on my '56 and liked it so well I bought another one for my project truck.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.