look in the workshop under your subscribed items, when you find it click on the button that says subscribed, that will unsubscribe you from the mod.

Before you do that though, make sure that you sell any items relating to the mod that are in your warehouse (i.e. body panels) and make you sell any cars that you have in your parking structure.

that is the normal proces i already knew :p but like my text said, the mod is gone in the list (workshop), but still visible in my game

Go to \Steam\steamapps\workshop\content\1190000 - that is where all the CMS2021 mods are stored locally. Within that folder you will see a number of sub-folders with numeric names - that's all of your subscribed mods. Look inside each of those subfolders, untill you find the one with the Mercedes (there should be a sumbnail image with the car). After you find it - delete this folder and you're done. IMPORTANT! Before deleting it, make sure, you sold in game all cars and body parts from this mod, or your save file may be corrupted, after you deleted the mod folder.

thanks ;)