Then one line of code needed to be added to the .gen file. So adding the mirrors was pretty easy, I just had a problem where the view in the mirror was squished so i just adjusted the unwrap and that fixed that.
Now for the Tachometer. For my car I've added an aftermarket tacho which is made by autometer. So I just needed to seperate the tacho's face just like the mirrors and unwrap it. But this time I actually needed to texture it which is what I did here.
TachometerRange=(100, 10000, 323, 420)
It worked ingame straight away and it seemed to be displaying the correct RPM.
I'll post a pic of it in game at a later date.