This have been discussed before, but I can't remember if there were a solution.
I believe the cause of the problem is from the heat of the bulb. Hot air absorb more moisture, which condensate on the cooler glass. The problem get bigger with cool air outside the glass.
Maybe try to drive without instrument-lights for a while?
Sorry, I have no solution, and I'm unable to test the theory myself as my P is still quietly in the garage with no engine...
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