I got a 99 Tahoe 4 dr 4X4 I have a major draw.

I got a 99 Tahoe 4 dr 4x4 I have a major draw on my battery. It is coming off the 50 amp lighting fuse in the underhood fuse panel. When pulling that fuse the ohms go infinite pretty much. With that fuse disconnected it also kicks out the fuel pump. I guess what im trying to ask is the 50a lighting fuse under the hood what circuit does it control or do you have a schematic for that fuse. I cant locate one anywhere, and im shooting at it in the dark now. Ive disconnected every fuse in car, disconnected sensors, lights, and no change in the draw on the negative. Any kind of schematic would help to further my search for the cause. One last thing is this truck use to be a paramedic truck, so lots of add on lighting that was removed, but so far have yet to find a draw in any of them systems. Please help otherwise the fuse is getting run to a toggle switch. So my wife can disconnect it if she has to drive it.