Hi, my name is***** your mechanic automotive expert, and I will be happy to assist you today. In some cases, I may need more information to assist better. I apologize for any delay.
Dear customer, you have very characteristic symptoms of problems at the starter motor level, the starter motor is an electric motor that is responsible for giving the first turn over of the ignition, so that the engine starts. When we turn the key and nothing happens, it is usually because the starter motor cannot turn over, this can be due to several factors:
It can be problems with the battery that has been used for a long time and must be replaced, but it can also be problems at the level of the battery connections or the starter motor connections, so it is good to start by checking here.
The starter motor works with a relay located under the engine compartment. Sometimes, the relay can be loose or defective causing it to sometimes not activate, so it is good to check that the starter relay is correct.
Finally, it can be problems at the starter motor level and your Mercedes is 2005, the starter motor is a part that is constantly subjected to turns over, over time the internal lubrication can lead the starter motor to get stuck at certain times that is why intermittent failure sometimes happens and sometimes doesn't.
The starter motor is completely replaced or its internal kit can also be replaced and a well maintenance can help to sort out the problem.
My goal is to give you the best advice i can, I hope to be helpful, if you have any doubt or want to know anything else please do not hesitate to reply back.
Kind regards, Sebastian.