Dunno if your switch is the same internally, but here's a how-to on cleaning one of these from a 1985 W126. (I checked on-line and the 190E switch casing sure looks identical though)
You can try shooting the contact cleaner through the opening in the case that you'll see when you pry off the knob for the function that doesn't work. The cleaner has a high alcohol (I think) content, which evaporates very quickly on the metal surfaces it finds. You might buy yourself some time, but eventually, the switch contacts will need R&R.....
http://www.peachparts.com/shopforum/showthread.php?t=135149&highlight=seat+switch