This could be a tricky one to find. I have checked for a wiring diagram however Merc being Merc don't release anything for Aftermarket use!
Obviously with their being no codes you have pretty much eliminated any electronic faults i.e sensors etc.
Without a diagram it will be back to basics and find out where the trigger wire that energises the starter motor solenoid via the ignition switch goes! It could be the ignition switch itself has a burnt out contact in the the start position.
If this isn't the case it will be trace the wiring. I'm not sure about Mercs but other vehicles I know of have a habit of routing the wiring through other things such as Junction Boxes (Fuse Boxes) which in effect act as a bridge between two separate looms on the vehicle.
I will assume that the garage you have taken it too have put their own live wire to the Trigger wire terminal on the starter in order to confirm that it isn't just the starter that is a fault and I will asume that while the key is help in the crank position they have checked for feed at the trigger wire!
If the faut lies in a broken wire within the loom it is often easier to bypass the broken bit and run your own wire!
I don't think I can offer any further help other than good luck in finding the fault.
Hope the information I have provided helps a little in tracing the problem.