I presume you are installing Comand-APS and Navigation ..
Check that the wiring loom for Navigation is there (in thr trunk), I've never done APS retrofit in an E class ... but I imagine you will also need to change the roof aerial to one with GPS receiver , and if the fibre is there in the boot will have to reconfigure it to add that pair to the loop (which I think is done in the right hand door sill duct, but again i'm not quite sure)
If you find the wiring loom to plug into the DVD nav unit in the boot, there will be a antenna cable going to the roof - (probably with a blue fakra plug on it), and if you put a multimeter on it, and it is not open circuit, (I guess it will be a few hundred ohms) then the GPS aerial is already there - if not, you'll have to change the antenna to one that includes GPS ..
Cheers
Richard