Regarding interior squeaking on S class cars, from my last one I found that using some silicone works well. The rear doors on my past car squeaked and I found lightly applying to the doorsills and underneath the respective matching surfaces worked well.
If the seatbelt area is squeaking it is often, in my experience from the seats pushing again the plastic seatbelt enclosures. This is cause by the seat cushion padding needing replaced. Once replaced it raises the whole cushion height and no longer rubs.
In the short-term you can use a light silicone spray can with a spray tube to shoot a blast into the offending areas. It is best, of course, to use a lot of rags to catch any drips and over spray.
Hope this helps,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)