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It could be steam from the heater core
It sounds like there may be a leak. If oil is leaking down on the manifold it will cause alot of smoke.
The blower resister is not supposed to be grounded. The smoke may be steam from a leaky heater core.
you need to replace your heater core that smoke and oily film is anti-freeze
Defective blower motor?
A Fire!
No, absolutely not. I am an experienced automotive technician and this is definitely not something that is possible.
The heater core has failed.
If it is smoke the heater fan motor has failedORElectronics in the heater have failed ORif it smells like anti freeze rather than smoke the heater core has failed.
In many cases leaves can cause smoke to blow through the defrost of a 1990 Ford F 150
it would cause overheating but the smoke is probally coming from some sort of water or antifreeze in combustion chamber
I feel these are unrelated but you can investigate a leak in the hoses leading to the heater core as they maybe leaking onto the exhaust pipe, that would produce white smoke, but not from within the tailpipe exhaust. Water and oil leaks will produce white smoke the smell will help you determine which it is.