Hey @XxQufDezxX,
I have quoted §3 - Save Editing, found in the official rules, below:
"Causing extreme lag or game crashes by save editing is not permitted. Extreme lag will not occur from duplicate parts unless the amount is truly excessive. Using truck parts on cars or buses. If you save edit to create a very long trailer combination but cannot safely handle it, you are liable to be banned for reckless driving and/or blocking and it is recommended that you remove the particular modification(s). If the Game Moderation Management thinks that your modification is not fit for our servers or negatively impacts other people, you are liable to be banned."
━━━
As long as you are abiding by the rules quoted above, you are allowed to save edit cabin accessories in your truck. For example, if you wish to place a Scania logo backlight in your Volvo, that wouldn't be an issue.
Mike