Since there is power in the system (at the heater fan), but you say even the main fan setting on the t-stat does not work, it sounds to me like no power is getting to the t-stat/switch. How about taking the thermostat out of the wall and checking for loose connections or broken wires at that point. Then use a test light to make sure that there is power available at the t-stat.
If power is on the input wire, find the wire output to the fan, carefully turn the fan switch on, and check for current on that output. If no current there, t-stat switch is the problem (no power getting through it).
If no power to the switch, then you may have to trace it all the way back to the power source, and somewhere along the line, there will be a break or disconnection. You might be able to narrow it down by testing in sections where access is easier. AT THIS POINT DISCONNECT YOUR POWER SOURCE. If you are not familiar with how to test this, get or make a long test lead with alligator clips on both ends, and hook it up to a volt meter in series. Then with one end connected to the power input wire at the t-stat switch, find the same wire at some point between the switch and the power source, disconnect that wire at that point, and hook the other alligator clip to the wire. Your meter should show continuity. If not, there is a break in that wire. If OK, then do the same thing on other sections until you find the break.
I know this may sound confusing if you're not a wiring guru, but the alternative is getting an electrician to do it - usually expensive. Hope this helps. And there may be someone else out there who can suggest other possibilities.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
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