1999 Mercury Mountaineer

You will have to remove both seats from the vehicle via the four bolts holding them to the floor. Once out, from the underside, you will have to remove the cushion and cover from both, switch out, and reinstall. If you're not mechanical, may be a little difficult, but doable. Make sure you disconnect any and all electrical connections under the seat before proceeding.

If you do this, you will still have one bad seat cushion but now on the passenger side. Have you looked at salvage yards or ebay sales? Many times they are available at reasonable cost and you only need to work on a single seat.