So my car was in an accident and needed a new trunk lid. I bought one from a junkyard, sanded, primed, painted, coated, and it came out great. My concern is that some of the primer overspray got onto the car's side and I removed it with 2000 grit sandpaper and water (bad idea, I know). Now, the car's side has swirl patterns and streak marks which I tried to remove with polishing compound, but to no avail. What is the easiest way to restore the shine?

Not the end of the world, by any stretch, so long as you haven't gone through the clear, which I'd think unlikely if you just used P2000 wet.

It will need cutting compound and a machine buff. If you don't have one then it would probably be cheaper/quicker/easier to take the car to a good detailer that specialises in paint correction. Most panel shops could do the same. Less than 30 minutes and you should be good to go.

Thank you very much, I will go that route. I'll let someone else do the detailing

Clay bar or a liquid clay bar product work great for removing overspray and won't screw up your finish, you would then finish it up with polish. On glass you could use reducer or lacquer thinner on a rag and or a razor scraper.