from my experance with the multidisk player they are not that good quality.
im 99.9% sure you cannot put another multidisk player in without changing your head unit.

The multidisks are a little fickle, some disks i put in work in one slot but not others, change slot and they are fine - its the same blooming lazer for crying out loud, run a lazer cleaner on it (disk with small brushes to clear any dust) first.

Are you getting disc error on the lcd desplay when you hit the src button for the multidisk. also are the ejecting ok as well?
if they are stuck then you will need to open the machine up and re align the thing, its a bit fiddly but there is an excellent guide on FF if you check.

If nothing is being shown on the lcd it may not be communicating with the head unit, check the connections and reseat them, if that fails try removing the battery / or fuse for the cd player for half an hour to effectively re boot the machine. this will hopefully pick up the multidisk changer.

I personally think the head units fait have supplied are excellent with many hidden features, but im personally glad i opted for the mp3 version over the multidisk.
hope that gives you some direction with getting it fixed
roy