i used mine going out as an opportunity to replace with a over the range convection microwave. i am glad they had not cut the external vent for the magic chef because it would not have matched up with the Apollo i installed.
although i haven't had any problems with the apollo, because of the reviews i read abt it, i cross my fingers each time i use it.
i believe the opening for my microwave is 24", the apollo is 22".
sense the origional post is old, jawaters may have already solved the problem. don't rule out pulling the shell off the microwave and see if there is a blown fuse in there. i know they blow for a reason but at work, they found that usually solved the problem.