When you're adding mirrors to a side view, I think a common temptation is to show all
of the mirror when in fact you're going to get a completely different view of it than if you were looking from a front 3/4 view. The resulting effect is a mirror that stretches further back than it should and looks almost as if it's folded in.
The best advice I can think of is to look at side views of actual cars and other drawings. Here are two examples:
I chose those two in particular because the mirrors are mounted in different ways (one to the body, one to the pillar). But you can plainly see the basic shape of the mirrors that both of them share, which is to say flat on the rearward (mirror) end and tapered to the front. Of course every mirror is different--trucks, for example, will have taller, more square mirrors--but the perspective stays essentially the same.
Another suggestion may be to check out some of the 360-degree views on some brand sites (BMW, Honda, whatever) to get a feel for how they look at different angles.
Hope this helps :/ In any event, no worries, that's an easy problem to fix.