I also get these two messages on my 2002 S60 T5. The position lamp is almost never failed, but if I play around with the harness connection at the rear lamps, the problem seems to go away for a while. As for the door open message, it comes and goes quickly, usually, I just see the overhead lamp turn on for a few seconds before I notice the message. I had the dealer look into it, but they did not find anything, nor were they aware of a more widespread problem.

The position light is probably one of the two bulbs in one of your tail light assemblies. I have a V70, but the setup is probably pretty similar. You'll be able to tell at night, or out of direct sunlight, but one of the tail lights won't look like the other. Good luck, as changing mine is a contortion and requires some patience. Take out the burnt one and match it. I carry one or two spares now, as they don't seem to last too long.

Had a door popping open (left rear) on my 2002 S60. Apparently this is a known problem (read about it on this or another forum). You didn't indicate if the door was actually opening or if it was just an indication error. Anyway, Volvo can pull out the defect from your car's computer easily and change the door lock(s) if required.
Cheers!