The Carousel has a backing plate that mounts to the wall. There are two diagonal(very long) bolts in the unit that captures the backing plate. They are accessed from under the two air filter screens on the underside.
There are also two bolts that come from the overhead cabinet to secure the front of the microwave/convection. Some cabinets have a false bottom that hides these two bolts and the false bottom will have to be cut to expose them. I used a 1&1/2 inch hole saw. With the help of a template that came with the new oven, the access holes came out perfectly aligned on top of the mounting bolts. I did not have to used the backing plate that came with the new unit...they were exactly the same. Sometimes, even a dog gets thrown a bone.
If you're planning on putting in a new unit, the Carousel comes with it's own template(incorporated in the box) that helps in the removal of the old unit and helps support the new one during installation...so don't damage the box that the unit comes in. The template IS the box after you cut out the template. Another set of helping hands is a big plus...that sucker is HEAVY!
One more thing...the backing plate or mounting plate, has a lip on the bottom edge and matches a channel on the backside lower edge of the unit. It makes the job easier if you take the diagonal bolts out first. After they are removed, the unit will rest on that lip, then the two front overhead bolts can be removed with the support of another set of hands. This is where the new box template comes in handy...you can just let the old unit down from the front into the 'cradle' of the template. Then the unit can be eased out of that back lip and set on the box. Same thing with the install of the new unit, in reverse.
Hope I haven't confused the issue, it really is easier than I can explain it.
Dang, really, just one more thing. Those diagonal bolts are held in place with a couple of 'star' washers. One of my bolts came out of the star washer (during shipping ??)and I couldn't line up the bolt...take it back down, secured and the rest of the install went well.
Check your bolts.