Mesh is transparent in solid preview, as well as in mat preview. It's only like that in edit mode and it disappears once mirror modifier is applied.
It looks like this, so it's not x-ray mode (⎇ AltZ). How to fix this issue?
As far as I can tell, it sounds like you’re a noting that the mirror modifier does not actually duplicate the mesh until it’s applied, but it does show you a wire frame of the pseudo-duplicated mash while you’re in edit mode (if the show in edit mode toggle button on the modifier is on) before you apply the modifier. This would be normal behavior.
If the modifier-created side of the car is still wireframe even when you are in object mode, that is a problem.