I've been buying harnesses from sites like etrailer.com. Just punch in the year, make, and model.
Not sure of the year, but if this is it, well worth $22 :
https://www.etrailer.com/Custom-Fit-Vehicle-Wiring/Chevrolet/Express+Van/2018/118392.html?VehicleID=20181170726
Even if you don't buy from them, they have installation instructions and videos that will point you towards the right wires.