bad wiring just like mine. my fix was to hot wire the switch and run wires from the switch to the headlights. you lose you high beams but it is a quick easy fix until you can get a new wiring harness.
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Check lead from fuse to Hi Lo switch. If you have voltage at the switch and neither Hi or Low work, the switch may be bad.
You might also want to look at the modification at http:/www.bigbroncos.com for improving the voltage to the headlights by directly feeding them from the battery rather than thru the voltage distribution system for the whole truck. It is under technical information.
I did a variation of that proceedure by getting a plow light harness. Details if you want them.