Ford F150 - p0135 02 sensor heater circuit malfunction

ok let me start at the begining, we are talking about a 2000 f150 4x4, 4.5L. I just had the engine and transmission replaced (not new but used from the junk yard) tranny 32,000 miles and engine 85,000 miles. drove it 1.5 hrs home and everything was fine it ran great shifted great. except for i noticed the floor vent by the drivers foot was blowing (ac knob turned to off position) faster you drive the more it blows, as you slow to a stop so does the vent. the mechanic was having trouble with check engine light and the overdrive light flashing but found a loose conection. do i have a wire harnes issue or a vacuum leak, faulty switch, what what what, im ready to burn this truck or drive it into the lake!!!!!

This ought to give you a few ideas of where to start: http://www.obd-codes.com/p0135