The trim plastic will have to come off to replace the backlight for the light switch...
The hissing could be a vacuum line that has come off. I would have it checked at once, for it can alter fuel mixture.
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM