Mike,
According to the 68 Pontiac Service Manual, on the 68 Tempest/LeMans/GTO the black is ground,(like you said)- they say there's a black with a double pink stripe that goes to the neg. coil terminal, and the grey (possibly your white wire?) goes to the "lamps" terminal on your fuse block for the light. they also show an in-line fuse in the double pink stripe coil wire.
On a 68 Firebird, they show a black (ground) wire, a brown (neg. coil terminal) wire with a fuse, and a grey (fuse block) lamp wire. I would guess that if you have just a black and brown wire, it's an unlighted tach.
Hope this helps you out.
Kurt
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