Do you still have the broken off square protrusion (maybe still in the back of the seat lever)? If so, you may be able to use a TINY amount of superglue such as 'plastic surgery' brand and re-attach the button without messing up the switch.
If you don't have the square section, I have repaired other plastic switches by inserting a small and short metal screw (a little bigger than eyeglass screws) into the broken off area, without going too deep and shorting out the switch. This leaves a peg like protrusion that a dab of 2 part epoxy on the back of the switch can grip. Takes patience and a steady hand. Good luck.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E