I had the opposite problem i.e. my rear door would often not unlock. So I followed a guide like the one above and took the skin off to gain access to the lock module, which was surprisingly easy to do. I then put everything back together after getting the part no. & giving it quick spray with WD40 and the lock has been fine since!
Either way, you need to take the skin off, which is really easy to do, and get a new lock module (from eBay or dealer) or maybe you'll get lucky like me & it just needs a clean!
Steve