Is it legal to write on a number plate?

[ncqtime]

I understand the name of the dealership needs to stay on but would it be legal to write 'are shit' with a black Sharpie after their name?

Probably. Just get a bumper sticker saying are shite with an arrow pointing to the dealership name.

Get a new plate without the shit dealer named on it.

Cover it in black tape

Why do you think the name of the dealership needs to stay on? It doesn’t, just get some new plates.