Battery constantly discharging

My opel astra h gtc 1.3cdti 90hp, constantly get battery discharge if i driving for like 30min my battery is full and i leave my car off for 3 hours or more i cannot turn on my car, batterry is empty full... I removed battery terminals (+/- cables) and leave car for a night, in the morning car started normaly so i think that something consume electricity while car is turned off, so i think that battery is ok. I think some ground cable is poor or some like that, question for help is, can someone tell me where is this ground cables are? Also i have multimedia screen radio installed and back rear camera but i dont think that is the problem because i install all of that very good? Any solution?