I don't remember the number - the one I got (the only one the parts store had) was a little too long but I bent the prongs in the housing to make it fit. There is a threadWhat type LED is required?
How did that thing build up so much resistance and heat? Well I know the answer to the resistance question, its all gunked up at the connections. But did you leave your trunk open for quite some time or possibly the trunk switch doesn't shut your light off as it should hence the burned lens and housing?
Lastly, why the heck are you getting so much water back there? I'd find out why that's happening rather soon, mine doesn't get dirty like that nor show signs of water intrusion.
As for the light/housing/lens, it should be some standard Ford part or something.
I have gotten into the (bad?) habit of leaving the trunk open when I get home (to cool the PEM while charging - I'm in Florida and don't have the luxury of an air-conditioned garage.)
The water bothered me at first also. I think the hole melted in the top of the housing and that part of the car (the outside/back of the trunk well) is pretty exposed to anything coming up from the road, so any water coming up will land on the top of the housing. Since the hole was there, it just leaked/poured in and doused the bulb and connections.