Fogged window on Shimano revoshift
It might be possible to remove, clean and replace the cover. As Carel says, know you can replace the part before disassembly.

The Dealer's Manual for the REVOSHIFT Shifter seems to indicate the method for removing the cover.
Page 14 tells how to remove the cover.   



Page 15 tells how to put it back on).



I left step 3 out for brevity. It tells how to remove and replace the cable.

I'd be tempted to leave it alone and shift by feel.