In order for any mods to work, even the allowed ones, they must be activated and put on in Singleplayer... If you want a trailer mod (custom trailer thing) follow these steps:
1. Activate in Singleplayer
2. Select origin and destination
3. Go pick up trailer
4. Save game
5. Exit and launch Multiplayer
Now for any truck customization like you said; you activated in singleplayer but can't remove it in multiplayer, that's just how it works for everything.. Any "allowed" mods have to be activated/deactivated in singleplayer.
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