Had the same problem with my Kangoo van 2005 plate.
Could'nt get the side door to unlock unless I kept pressing the key lock/unlock buttons took about 5 minutes to unlock it. Having read the above posts I then removed the triple striker plate from the door surround (just behind the passenger seat) and pulled the white wire from the spade end and taped it up.
replaced the striker plate unit and closed the sliding door, pressed the lock button on the key and all the doors locked except the sliding door.
I then repeated what I had done previously, replaced the white wire by pushing the spade end in place then pulled the yellow wire off the spade taped it over and put it out of the way. Replaced the striker plate, locked the door with the key button and hey presto all the doors now lock and unlock OK. Of course they are not locked when I remove the filler cap but as someone else pointed out it's a van and no one else is going to open the side door when I'm refueling.
Taviman
September 2012