2000 Ford Escort No start, mechanics baffled!!

I've recently posted about a no start problem. Current situation is no start. There are no noises, clicks or anything when trying to turn the key. The "Service Engine" light is dimmly lit at all times. It only brightens when you turn the key. There are no interior lights, headlights, or any other electrical components that are operating.

The mechanic I had service the car ran a diagnostic test which revealed no problems at all. The starter and battery were good when I took it in, but they replaced the battery with a new one. The weird thing is the the car started right up with the new battery for several days before returning to the state previously mentioned. They felt it could be an electrical draw, and installed a quick disconnect switch to the negative terminal to prevent draw on the battery.

I disconnected the negative terminal for 3hrs and when I returned, it wouldn't start. I'm no expert but I can't see a brand new battery being drained when it wasn't connected. There are no anti-theft systems. I'm lost. Could there be a problem with the positive battery cable itself? Does anyone have any other ideas. I'm ready to take the car to Ford and tell them to rip the damn thing apart.