re your 2002 laguna 1.9L dci
from your posts you have made so far it looks like you have had water get into the electrics (fusebox and
) behind the dash -between the steering column and the end of the dashboard. the water probably came down the front door pillar and has gotten into the electrics down that side.
you do not need to replace the rear door locks!!!
(the garage is chasing the wrong fault and is going to waste a lot of your money)
shorting out cos its PCB is soaking wet!!! just needs drying out and cleaning.
did you get the blocked sunroof drainage holes sorted out yet? sort that out first!
to fix you need 3-4m of netcurtain rail wire(from a ironmongers) only about £1 or so. fully open the sunroof, run the wire down the pipes and push through the gunk at the bottom of the pipes, the end flaps get hard with age and block up, you may have to remove the inner arch rear covers to get at the endcaps and take them off and bin them!
run hot soapy water down the pipes once they are clear to get the gunk out.
(dont forget to do the rear drain holes as well, they will need cleaning as well, thats why u need 3-4m of wire)
im not sure quite how to deal with the damp/wet fusebox issue, dry the car out or remove the affected parts out?
stick a large dehumidifier in the back of the car to draw out the water and a small 2-3kw fan heater in the drivers footwell pointed up towards the fusebox to dry the electrics out. dont get it too hot or you might melt some plastic under there.
(you will need to remove the under dash kick panel(it unclips and has velcro fixings)
it may take a few days at least to draw out the water
worst case the garage will have to remove the dash to remove the electrical boxes and disassemble them and wash them with isoproponal alcohol to get any damp dust and gunk off the PCB's, once cleaned and dried use conformal coating laquer to seal the PCB's from further shorting out by damp or dust in future.
(though they are more likely to replace it and try charging you £500+ for them not fixing the problem the cheapest way)
best of luck