If the new switch doesn't sort it you may have to check the wiring back to the BCM to check for short to earth

Door Ajar Indicator Operation
The body control module(BCM) receive discrete inputs from each of the door ajar switch signal circuits.
The door ajar switches are integral to each door latch assembly
with a separate switch on the sliding and cargo door models.
When a door is opened, the normally open door ajar switch closes.
With the door ajar switch closed, ground is provided to the door ajar switch signal circuit.
The BCM interprets this and communicates via serial data with the instrument panel cluster

 

Ye i dont think the switch will fix the issue as iv also looped it with a wire which in theory should work so looks like i need to trace it back oddly no short detected by diagnostic

 

Check the wiring in the rubber boots that run from van in to doors, sounds like it could have broken wire in there to me.

 

Put a new sensor no go will do some continuity test on cabling and uodate thanks guys for your help

 

So checked all circuits iv given up you get 4v when pushing the switch in so it is working

 

any luck mus22 ?
have the same rear door ajar issue as you on my 2017 swb vivaro
although mine will lock the alarm will go off at random times
done pretty much same diag as you

the post from valer describes the door ajar switch being part of the door latch
so had a nose at my one an noticed there is no spring back to it and some of the latch plastic covering has peeled and gone inside the latch itself
managed to pry out a small piece which could be the issue 🤷‍♂️
will remove latch on weekend and have a look
poss may need to replace
will update any findings 👍