Low beams and back door locks S80 2001

you did not list year.
google ( insert year) volvo xc90 fuse diagram

It is likely the head light switch that is defective.

It can be faulty low beam lamp or its connection issue for low beam lamps.

There are few solved helps links with proper solution to troubleshoot these issues.I suggest you to refer the links below.Click the link:----
Passenger side low beam out?

http://repairhelpcenter.blogspot.com/2012/01/passenger-side-low-beam-out.html

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Low beam HID light goes out? http://schematicsdiagram.blogspot.com/2012/01/low-beam-hid-light-goes-out.html
----------2007 Dodge Caliber: Low Beam Headlight Not working http://technoanswers.blogspot.in/2012/06/2007-dodge-caliber-low-beam-headlight.html
-------------Chevy Tahoe: Low Beam Lamps are not working? http://technoanswers.blogspot.in/2012/04/chevy-tahoe-low-beam-lamps-are-not.html
----------2001 Chevrolet Tahoe: Both Low Beam Lamps Not Working? http://technoanswers.blogspot.in/2012/04/2001-chevrolet-tahoe-both-low-beam.html
--------------2002 Toyota Tacoma: Low Beams not working? http://technoanswers.blogspot.in/2012/04/2002-toyota-tacoma-low-beams-not.html
-----------Low beam headlights Modification? http://schematicsdiagram.blogspot.in/2012/04/low-beam-headlights-modification.html
-----------DRL and Low beams not working? http://schematicsdiagram.blogspot.in/2012/04/drl-and-low-beams-not-working.html
-----------These will help.Thanks.

replace high/low swithch

I suppose my clarification was more of a possible solution. Looking for other thoughts or ideas as well. I can live without the high beams. I can't live without my low beams!

i think im right in saying the low beam bulbs are twin filament(low and high beam in one)and the main/high beam are single filament(spotlights).

FIRST THE HEADLIGHT SWITCH??? AND CHECK THETURN SIGNAL MECH

If i am correct, You have a fuse for low beams...Silly huh....... Well give it a check and you will se that you ahve fuse for high beams and low beams. Good luck.

faulty light switch or light control module

dim lights are generally bad ground / rusty -loose ground points or wires