You need to sell them off first. Then unload the mod. Otherwise, you'll have errors in your save that might stop it from loading.

Thx Mardoin :-) What happens then if a mod disappears from Steam? Can I remove the vehicle than or how can I avoid problems? Would it be better to not use any mods for that reason?

That almost never happens. I've rarely ever seen a mod get taken down after it's been on Workshop for a while. I wouldn't worry so much about that really. But, if it did happen, you'd get a description of the mod car in your parking as being undefined. And, you wouldn't be able to do anything with it. Trying to bring it into your garage would probably lead to crashing / freezing. In the last version, there was a mod that would enable you to move it from parking to the mods que and then you could delete it. Was called 'Parking Organizer' but, I'm not sure if it was revised to work with this game version. You'd have to dig through the modding subforum. You probably should make a manual backup of your 'Profile' folders once in a while. That way... should something like this happen, you have a backup in case it gets corrupted. Also, don't rely on that Steam Cloud sync for that purpose. It's just as likely to cause a corrupted save file anyway. Turn it off... at least for this game. I turn it off globally but, that's just my personal preference.

Many thx for your polite and detailed answer. Yes understand that better now :-)