I believe the alarm controller is under the panel ahead of the passenger footwell. I'm pretty sure, though, that you don't have to get into that area to wire your RKE to work with the alarm. Check mb.braingears.com for a wiring diagram. This link might work -
http://mb.braingears.com/124_DISC1/Program/ETM/ETM.pdf
It's an 80MB file so be patient.
Sixto