If your seat bottoms have pulled loose of the cushion, here’s what you need to know about reattaching them. The process is not complex and can be completed with common hand tools. Start by disconnecting the battery and waiting 15 minutes before removing the seats from the car so you do not trip the Airbag light (and have to make a trip to the dealer to get it reset). Use a Torx 40 bit to remove the four seat bolts, and disconnect the electrical connectors under the seat. Do not reconnect the battery until the seats are plugged back in again.
Place the seat on its side with the seat height adjuster lever facing up. Use a pry tool to remove the center circle, and then use a hex key to remove the lever. Starting at the front pull to release the side trim (numbers 2 and 3 in the diagram below). There are two press-in connectors (number 9) and one plug (number 7) to remove. Switch over to the other side and remove the other piece of trim.
Working under the seat, look at the long pieces of black plastic that attaches the seat cover to the front and rear of the seat frame. This plastic is very brittle and care must be taken to free the cushion without breaking it. Look at the sides of the seat and gently remove the parts that wrap around the bottom. The cushion should now come free of the frame. Keep track of the routing of wiring for the seat-heater and the seat occupancy sensor (on 2005-2006 models).
Looking at the bottom of the seat cushion, notice the wire frame that is embedded in the foam. The first image shows the problem most clearly (though not a MINI seat.) There is a frame sewn into the seat cover that can become detached from the cushion over time. The second photo shows this structure from a MINI seat upon which the Seat material must be reattached. To do this, you will need a set of hog ring pliers and clips. In the MINI, the seat material attaches in four places on each side of the cushion. Installation of the cushion is the reverse of removal. Reconnect the electrical connections before reconnecting the battery and be sure to torque the seat bolts to 26 ft lbs of torque.