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.