Water Leakage Testing for rooftop air handlers?

Looking for any helpful information. I'm a CxA and we have a project with 11 custom modular air handling units that are installed on the roof of the building. During testing we noticed water build up in the cabinets after rainfall. Since then, the construction team has been unable to correct the issue. The unit manufacturer has been onsite and conducted their own "hose spray" testing, and has replaced door seals (and even doors) but after last week there is still water intruding into the unit.

The client is now asking if our firm can write up a testing procedure for conducting a water leakage test, identify leakage points and assist the team with correcting them.

We're well versed with envelope testing so this isn't exactly completely new, but I tried googling the situation and am surprised there's little to no information already out there.

Has anyone else dealt with this? Are there any pre-written testing standards for this?