Gas gauge does not work properly, the speedometer doesn't work and the bright headlights don't work?

I bought this truck a couple of weeks ago and the gas gauge does not work properly, the speedometer doesn't work, and the bright headlights don't work. When I use my digital diagnostic tool, it says it's running too lean and there's a problem with the ECM. It's impossible to reset my ECM? I've checked all my fuses and there was none blown and there is no specific fuse for the speedometer or the gas gauge just the dash cluster and the fuse isn't blown. Please help. Also, my driver side power window switch doesn't work so I bought a new driver side power window switch. The driver side power window still does not work. Does that mean that the problem would be in the wiring harness that goes from the window switch?