The same thing happened to me on the drivers side and it turned out to be caused by a kinked up sunroof drain hose. I de-kinked it and problem solved! You can examine this by removiing the a pilar. To do this you remove the plastic airbag badge and unscrew the screw behind it. Then just pull the a pillar off carefully. This would be much easier than removing the door panel so you may want to check that first.