I have recently installed a Toad alarm to my Mb6, I found a lot of conflicting info on the central locking activation for an aftermarket unit.
It is difficult to interpret and manipulate the locking signals due to switches in the locks themselves providing feedback signals. To clear this up and keep all the door locks working as they should, also retaining the use of the original fob, the simplest way is to find the central locking module located behind the heating controls (centre console), open up the unit and attach the negative trigger system directly to the pins on the dual relay as shown. these pins will activate the door lock/un-lock when grounded.
Hope this helps.