can i use the oem key fob to operate an aftermarket alarm system???

Might want to call DEI to see if this will work with the Mercedes system
http://www.drdetailshop.com/700t.htm

I have seen some Mercs with the Clifford System added on.

As an installer Ill tell you its not worth the price to upgrade to that factory security... the top reasons people add factory security to there vehicles is to get Keyless entry, insurance rebate or because its in combination with a remote start system. even adding on an aftermarket security system would not help the vehicle... For the most part the ML55 has a pretty secure system as a whole. The only advantage might be something like a glass break sensor but even that is already in some of the ML55s. If someone wants something in your car they will get it. From my experience those who target MB, BMW, etc are not your common punk trying to steal a radio... if a thief wants your vehicle or something in your vehicle then they will get it. An aftermarket alarm works only so well... afterall the car could be towed to a location where the person has the ability to access it at there own convenience with more than a handfull of tools. There is no security system on the market that is impenitrable... even Lo jack and On star can be disabled- I have seen it in demonstrations... IMO save the 500 or so your gonna get charged for an install and parts and put it to use somewhere else...