When the passenger door is open - the red light inside the door is not coming on nor does the "open door warning light'' appear on the dash. Further, as you can guess, the interior light (with its switch in the "DOOR" position) does not come on when this door is open. Can I fix it myself? - what's the problem? Help pls! (All other doors behave as they should).

It might be obvious but have you checked the bulb?

quick check or diagrams and it can only really be that.

or the door switch

sounds like door switch to me, tis the only thing it can be.. take it out and see if any crap is on it, clean it up and put it back in..

Assuming all is ok with the bulb (and the drivers door light comes on), have you checked the door switch? This goes back to the ECU separately so could either be the switch or the ECU.