There is a clicking sound in my passenger door when I open and close the driver door, sometimes.  I assume it must be a relay for the power steering or central locking but was wondering if anyone knew what it was before I continue my investigations.  As it is a ventriloquist, I started off thinking it was the alarm system and then the AC system as I have been having problems with those.  However, I have narrowed it down to the passenger door.  I just don't know why it makes such a noise.
 
Any ideas?

It may be the piece that hold the door from opening too far.
I have found that these often dry out and they go click when you open and close the door.
You could try WD40 or grease etc to see if this helps.
You can find out if it is this particular problem by removing the pin that holds the piece to the door and seeing if the  clicking stops.
Take care not to let the door swing too far.
If this is the case and the WD40 does not eliminate the click you may have to take the door card off and grease it from the inside.

Thanks toimr2.
 
With both doors open and the ignition on, if you press the door switch for the interior light on the driver side, a relay in the passenger side clicks on and off twice.  It does not do it when I press the door switch for the light on the passenger side.  Therefore it is not mechanical and it is not related to the light as the light remained on when I had both doors open and was just working the switches.
 
I'm realing starting to think window relay but I find it weird that it is actuated by the driver side door switch.
 
Update: The book shows it to be the Door Lock Control Relay which starts to make some sense.  I'll have to study the circuit diagram to see why I am getting the double clicking.

Update 2
The ignition doesn't have to be on.  The key just has to be in it.  Funny little cars!