This is not a simple system to diagnose. There are a few different switches that could cause this problem. The first thing I would do is verify that the door jamb contact plates are clean, dry and, corrosion free. From this point though you begin investigating in the door itself and some of this testing will require a class 2 scan tool like the dealer has. See this type of problem actually can produce a trouble code which takes all of the guessing out of play. Here is a wiring schematic of the system so that you can see how advanced the system really is.
Keep in mind that depending on how many miles you have, you may be able to get this repaired under a factory warranty. Feel free to contact me with any questions! Good luck!