Berta...these type indicators use a rolled-on bezel assembly which requires a special fixture and jig (not to mention another new bezel ring..the silver band).
I had the same problem when I first got the bike. I used some of the smallest and thinnest sticky-back foam insulation strip I could find and stuck it on the backside on the bezel of the Tach (sandwiched between the cup). I was more annoyed with the Tach rotating off-center while riding...but the insulation strip cured that also.
I haven't had a fogging-up problem since, but that could just be coincidence...I think there's something to what Pat said about the problem stopping with time (dirt, dust, soap, sap...sticky things winding up in the crevices to seal them.