Left, from the rear facing forward
Did you look under the sound barrier under the rear seat on the left side? I don't know for sure if your 190 is designed like my 300SD, but that's where I found it. I obviously looked over it twice before realizing it was right in front of me, and unplugged it to eliminate the false alarms.
Worked like a charm
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan