The Longview # CTM2200A slide on custom towing mirror is a confirmed fit for the 1997 Mercury Mountaineer, as long as the factory mirror looks like the one shown in the photo. It doesn't fit the mirrors with the puddle lights, as well.
The mirrors are sold in a quantity of 2, one for the passenger and one for the driver's side. By clicking the provided link, you can see a product demonstration video featuring the mirrors. view full answer...