its true bo it could just be the selenoid is stuck. its definately not a bad ground because it is grounded as soon as it is bolted in there is no ground wire. i learned that because i tested mine before i installed it and wondered why it would not work. i imagine it is the same for your mercedes. on my mercedes there is only two wires the power to the selenoid which is the big one and the small one which is the power to ignition switch. check and see if any of these wire are broken i had a wire which is installed on the starter and does not touch the car( it is a industrial wire going from selenoid broken and they are hard to repair) so check all of that and e-mail me with results. I know how much of a pain in the ass installing and problem solving this is so i will help you to my best of ability.
tom_williams@canada.com
Cheers and good luck