If you can get access to the PSE pump, you may be able to create the vacuum to activate the boot. Worked on a W210 when I locked the keys in.
I don't drive an MB so don't mind giving away clues
In any case you need to get into the car first.
Often the luck is just crudded up and a good soaking with WD40 can help. Premium manufactuers like Volvo, put a cover in place to stop this happening...