You might have to cut somewhere to allow for airflow from the trunk area to the cabin area. What model is it? They sometimes build these cars with double steel wall in order to suppress outside noise. Try to take some pics of the trunk area if you can. I've worked on a S-Class before and these cars are a bit tougher to run wires in comparison to say like a Honda. There should be a factory amplifer towards the rear, left hand side or rear right hand side quarter panel where you can access the signal for the RCA, if not you can just get the signal from the rear speakers. Hopefully there are others out there that have more experience with Mercedes and can give some more insight. Good luck with your install man. 
