Yes it is but not all the parts are. Most Auto & Truck manufacturers depend on smaller companies to build the bits and pieces that make up a complete vehicle. Keep in mind all these parts are made to the manufacturers design specifications. Some examples are ignition components,computers,wheels,glass, bearings, ETC. 

I'm sure you have heard of  AC-Delco, NGK, Nippon-Denso ,Champion,Holley ,Carter,bosch,ETC. AC-Delco has been building GM's ignition components for many years and they still are but under a new name of DEL-FI (not spelled correctly but you get the point.) DEL-FI has branched out and is building a lot of the parts for all of the vehicles built here in the U.S. they also provide replacement parts to the different Part supply chain stores in the U.S. (like NAPA). So to say Mercedes-Benz is a 100% German Car the answer is yes but only in design not in the sum of all it's parts. Hope this has help -Mike