Not sure how similar the 115 is to a 108 in regards to the heater controls, but you may have to have the instrument cluster and glove box out. On the 108, with both item removed you gain access to two 10mm nuts that retain the facia for the heater control.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.