Its a brand-new OEM light mate, purchased it in the hope it would work correct, but it does exactly the same as it did with the original one.Hmm - that's frustrating.
Do you have access to another dome light which you know is working (or another Sprinter / Crafter you can try installing your dome light into)?
That would confirm if the fault is in the dome light itself or in the connection to it.
I would offer to post you mine - but I suspect the postage costs to send it to Finland and back to the UK would make it impractical.
I will go and try this right now on both front doors, I have both on hand, compressed air, and WD, so will report back.Spray a healthy dose of wd40 into the driver side lock and then use compressed air to blow it all out. That makes the door sensor work sometimes