THis is the most interesting electrical failure in one of these cars I've heard about - and I've been an owner for almost ten years now.

So, what you are saying is that something failed such that power is still fed to:

• The ECM via the engine control relay.
• The Fuel Pump via the fuel pump relay.

Who knows what else. I"m having a hard time seeing how the light switch could lead to this. But I do wonder about the security pump/module.
I'd yank the trim from the trunk area and look for moisture/water in there. Then I'd grab the service manual and try to find the connection here - I tried, but I dont see it.

This is a good one.