Hi Andy,
The washer jet is indeed built in to the light.
Sadly, it comes as a complete unit (lens, washer jet and LED lights).
I believe all 2015+ tailgate vans have the same high level brake light unit, as mine came without a back window but still has a washer jet (although not connected).
Here’s one on eBay, but it doesn’t list the part number:
https://www.ebay.co.uk/itm/174330329633 ... media=COPY
The whole unit is a push fit into the top of the tailgate, with electrical and hose connection behind. They’re very tricky to remove without breaking the lens, as you’ve found out!
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