Phase 1Go to them while they're together and say "Listen. I'm not stupid, I can tell someone is opening my drawers and going through my stuff every time I'm gone. I don't know who it is, and I don't care. Stay out of my room. The next time you feel the need to be a creep, do it somewhere else. I'm willing to overlook it if you never do it again."
They will likely respond with some denial of knowing what you mean, just say "Stop lying. I came and told you how it is directly like a decent person. Now respect my privacy."
Do not under any circumstances reveal the camera. If they say "How can you be so sure?" then reply with something like "Like I said, I'm not stupid. I can tell. If I went through your stuff all the time, you'd probably notice too." The fact they don't know about the camera is what gives you the advantage, and ensures you will catch them if they do it again, because they cannot possibly account for it in any plan they make. Knowledge is power.
Phase 2After the first phase, they may continue their behavior, but try to be especially discreet. If this happens, you've already tried being direct and found they will not stop unless they want to, now it's time to make them want to.
Begin this by doing something which negatively impacts them, such as stealing an item from their room and throwing it in a garbage can somewhere (off your property of course, like McDonalds). It has to be something which they cannot prove you are doing.
Then, every time they snoop through your room, do another negative thing like this. Keep doing it every time they repeat themselves.
At some point, maybe even after the first time, they will come to you about it. Like, "Do you know what happened to my stuff?" Just reply to them, "I don't know. By the way, any idea why someone's still going through my room when I'm gone?" or "I'm not sure, maybe it's the same mysterious person that's still snooping in my room all the time."
This will catch them off-guard, and of course they will make the connection that maybe you did this to them, and maybe it was because they did something wrong. No matter what, deny any knowledge of anything bad you've done, but continue doing such things every time they go in your room.
This will likely make them stop, and while they continue to do it, you will get some satisfaction from getting back at them.