Is there any aftermarket equip on it? Look there first.

Then look into doing a voltage drop test on the car, systematically removing the fuses until the suspect system is found. From there look for an obvious malfunction in a component. Every car line I have worked on has this issue at some point, BMWs are hell with it, Mercedes are better, all have it.

hit me back if ya need anymore advice.

just noticed the date, oops. the same advice goes for any car though