if you have unpluged the passenger side motor and everything other than the passenger side works, then the passenger side motor is the one you need to be looking at sorting

but the passenger side is also a "master" and not a slave so it just takes time and effort rather than throwing parts at it to solve this issue

had this on the fiesta when only one side will disarm the alarm but both sides will arm it, couldn't ba arsed after several hours trying to get it to work

there will probably be an issue with the cable that controls the lock or the little plastic widgets that hold everything in place, they can become brittle over time and if they've broken you just need a replacement

be warned that it is a bit of a p[ain in the arse to do though but remove the entire locking mechanism, carefully, and take pics of each part so you know how it goes back together again

i'd suggest getting hold of a decent set of phillips screwdrivers to undo some of the screws as you don't want them to get stuck or round the heads off