Open the trunk. You will need to remove the floor mat, the spare tire cover, and the cargo area carpeting on the right side. In many W210s, there is a storage cubby on the right side of the trunk; the alarm is located essentially behind this area, towards the rear wheel.
If your alarm is going off randomly or making a faint "chirping" noise, the problem is likely the siren. w210 alarm module location
There are two primary reasons for this W210 alarm module location: Open the trunk