According to this diagram the eighth wire is for the park lights. The way I read this you don't need to replace the front sockets with dual filament. It will light the single filament bulbs when the headlight switch is in park position or flash the appropriate light when in 'turn'.

Pretty nifty switch, I'd say.

"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.