If I recall right it should be a green wire and I think it will go to a bulkhead connector at the firewall by the brake booster. Maybe start there and see if you find a green wire there? It may have some of that braided type of insulation over it.

Do you have a service manual for your car?  It'll have a wiring diagram which should show where it goes through the bulkhead.   Where is the sensor?  Drivers side head?  Passenger side head?  Intake manifold?  As Carl indicates, it should likely be a green wire, and if I recall the sensor is on the intake manifold.   You may need to peel a little of the loom wrapping near the wiper motor to see what you can find.

yep, green wire, braided cover 

'68 was the first year temp unit was installed in cyl head

this is a camaro pict, but you can see the wire coming out of the alt harness

Yours looks a little different from mine. . My alternator plug has two wires while yours only has one. . To make matters worse, , I'm color blind. . Of the three wires going to the alternator, one large looks red, one medium looks blue and the small one I can not tell. . The small one goes to the side of the plug marked  R  . .

. . Does your alternator have two or three wires ? ?  Mine has three. . I don't see colors well. . One looks red, one looks blue and the small one ? ?