Run out and get a step drill.
Lowe's or Home Depot have them. They are really for sheet metal but do great on fiberglass. Leave a nice clean hole and wont chip the paint around it.
What about attaching it by the bracket under your mirror, then you shouldn't have to drill any other holes. I don't know how the mount is for the light but you may even be able to modify it slightly to do it if you have to.
Todd
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