My 91 is smoking out the vents

What did the smoke smell like?

Not having been there, it's hard to tell - BUT, is it possible that it was actually water vapor? Under certain conditions (warm, humid outside air being drawn in and cooled to just the right temperature) the moisture in the air condenses into tiny droplets and forms a mist, exactly like how clouds form.

I've seen it happen in my '93 Villager, although never in my Mountaineer. It used to happen when the cabin air had been chilled down pretty cold using MAX A/C, and then I switched the A/C to draw in outside air.

Most likely it's a blown heater core. It may look and smell like smoke but it's probably just hot coolant in the form of steam.

The whole thing now makes sense since the resistor that most likely burned out is in the blower housing. From under the hood the resistor is located inside the housing below the blower motor.

Steve

Absolutely right. The blower motor resistor pack, inside the air manifold under the hood. There will be a 3 or 4 pin connector going to the pack and it pulls out of the manifold.

Don't let him take the a/c unit out of the dash! There's no need. Just get him to check the fan speed resistor pack as stated by the two other posters in this thread. You probably blew two of the resistors and just need to replace the board. I hope you don't get soaked.