The point of the return policy is that you can buy something without worrying to much that you'll get it home and use it and realize you don't like it.

It would make more sense to me if you just drove back right now, returned the light that hasn't worked well, and bought the new light. If you're thinking about ethics just buy the new light at full price. I mean they have the return policy for a reason.

I would never return something (unless it was still brand new inside it's original packaging) in order to save a few bucks, but I absolutely would return something that had worked poorly - that's why they have the return policy.