I'm not sure about the '86, but in my '89, when the defroster is on, the AC comes on to help disperse the moisture and humidity in the car.
If your AC isn't working, that's probably the reason you're experiencing the fogging problem. Just drive with your window cracked, that should help.
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'89 LX 5.0 - 5 speed. Original Owner
'94 E-150 4.9 Hi-Top Conversion Van
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