Most likely wire harness issue...but your guy does not seem to be E320 experienced, so I mention two VERY common mistakes Tech/Diy make on a head gasket change on this engine, ..............just to be on the safe side.
One is the tensioner has to be reset before re-install and the other is the valve timing on that engine is Variable Intake valve and there is a specific procedure to set it.
Here is some good reading addressing both and you may want to print it out for info in the event your harness is not the prolem [ but most likely is]
Might also just want to show your guy this info just for re-assurance, specially the tensioner reset..failure to reset that can lead to broken camshafts , real quick. He may well be aware of both of these 104 specific procedures, but we see many good Techs who are not, simply b/c they have not done one of this type before. If he was not aware of the specific harness issue on this chassis, he may also not be aware of these , being 104 engine specific...[ not to imply he is not a good tech]
http://www.peachparts.com/Wikka/M104HeadGasket
Better that you know.