It could be the headlight switch itself, do they work with the bypass switch? I had the same issue and it ended up being the headlight switch itself. You can check vacuum with a $20 vacuum tester at each location just be sure to plug off the lines to the wiper door system first. You want to separate the 2 systems when trouble shooting or you end up chasing some ghosts.
Also on high compression engines your vacuum can run pretty low out of the manifold which can cause some issues, I actually cannot run the inline filter as the vacuum drops too low and neither the wiper door or headlights work.
You can purchase the trouble shooting guide online which is a huge help when your having a problem with the vacuum system, its very detailed and accurate.
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