They keep breaking into my car!!

[Sil80.Rb.Power]

So i hide my craftsman tool box set that i just bought my self this Christmas in my car and my expensive welding supply's. So when ever I'm at school or go out my brother can get it my car somehow and his friends s13 key fits and turns on my car. i lock my car and my stuff but they always get in and get my stuff .. what can i do to prevent them from getting in my car. i want to get an alarm but i know if i get an alarm they'll still get in and just disconnect the negative battery cable to stop the ringing. Whats a way/affordable that i can do to prevent from getting into my car. Ive told them before not to get in my car but im going snowboarding with my gf this weekend and i know they'll be getting in my car when im not there and maybe even joy riding on my gas.... what can i do !