Im not sure about Mercs...but I KNOW that ford focus can be broken into by shorting the brakelight, which shorts the cars electrics and opens the doors. Me and a mate tried it on his ST170...and it worked... :|
Needless to say he sold the car 2 months later.
I had an old J reg 3 series broken into........ they disabled the alarm through the side indicators on the wing...an old car and well behind the times in terms of alarm protection...