I know that the line you are seeking runs off of manifold vacumm from the rear port near the PCV valve. I believe it was a steel line that runs down the manifold along the left( driver side ) bank to the fuel pump. It has to be manifold vacuum not ported vacuum. Anyone that owned any sort of car with vac wipers can confirm that because if you were not on the throttle those vac wipers were not doing much for ya'. I looked for a picture of the setup but couldnt find a good one. Gordy Chilson has a 69 390 AMX (BBB) that has vac wipers. If you can get in touch with him he could tell you all about it. The only thing I am using for reference is my brain from my old reliable 71 Matador w/304, pretty sure I still have the lines tucked away somewhere. I hope this helps you and maybe somebody can shed more light on this for you.