The macerator may be jammed by solids or just lack of use, in which case it would need freeing. The book says:
"After long periods of non-use, a stuck impeller can be easily broken loose with a screwdriver inserted in the motor shaft slot. Remove the rubber boot, turn the shaft clockwise as shown, and replace the rubber boot."
Usually when it's jammed, the fuse also blows. And it should be the fuse on circuit #9, in the 12-Volt Distribution Panel under the rear sofa.
Hope you can get it running.