button to empty holding tank on princess v39 / v40

If I remember correctly it is on the main fuse/switch panel in the overhead locker in the galley or saloon, its a while since I was in one.
 
Try and find the macerator then trace the wiring back to the switch if possible. If it's not a factory fitted it could be anywhere but try looking in bilges. Tracing back from macerator is best bet though.
 
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