Cab clearance light issue

I would put money on the ground wire is corroded from the water intrusion.
Using the metal dash frame as a ground attach the test light to metal frame and touch ground wire up next the cab lights.
should come on

 

You can clean those harness Connectors with with vinegar and baking soda mix in a small cup and dip harness ends in the cup that will clean the corrosion then spray brake clearner and blow them clean & dry with compressed air they spray them with contact clearner and put some dieoelectric grease on the Connectors and plug them back together that should work

 

We use and maybe overuse things like dielectric grease and anti-seize. We replaced the cab marker lights on our 378 with LEDs that had housings similar to factory. We had a bugger of a time getting them all to work at first, when we had the headliner down. I believe our issue was too much dielectric on the bullet connectors, and the LED lights didn't draw enough current to overcome the dielectric grease. Once we wiped them off good they worked fine and have since then (now three years later). This probably isn't the OP's issue, but certainly something to keep in mind.

Edit to add: There was still a film of dielectric grease on the bullet connectors, we just wiped the excess off. Dielectric grease is not conductive, but it sure works well to combat corrosion.

 

It's a little hard to tell the size based on pictures. The width of the pin correlates to the connector size: 1.5mm = 150, 2.8mm = 280, etc
The small 2 pin connector looks like a metripack 150
Page 58 (page 4 pf the PDF):
https://docs.rs-online.com/f13f/0900766b814c67ea.pdf

The larger 16 pin connector looks like metripack GT series, also 150
Page 13 (page 4 of the PDF):
https://www.powerandsignal.com/docs/catalog/gt150.pdf

Measure the width of the pin to make sure. Terminals come in a couple sizes to accomodate various wire sizes, usually the larger size is what you need. Mouser, digikey, or other places have them, just search for the terminal part number. I don't have any of the GT series terminals and only a few of the regular metripacks otherwise I'd send you some

Might as well order some of each, male and female, they'll be handy to have for the next time this happens. Also get a crimper that will make a proper crimp. All the connector companies sell a specific crimper for their terminals for $$$, but one of these will work fine with a little practice:
HT-11 General Purpose W Crimp Tool 1 Each [HT-11] - $43.66 : TheElectricalDepot.com!, Electrical Connectors and Much More!