My car lock often will not open the door

My car lock often will not open the door. I believe the battery runs down when it sits overnight, although there is nothing to drain the battery. How can I determine what is causing the problem. I know it is not the battery, it's new. This was first a problem, after I had a new stereo put in about 3 years ago and it has continued. The car always starts, but the lock is a big problem.