I would assume the 3 dim wires would go to the 3 contacts on a potentiometer. That diagram isn't showing the correct location where the words DIM4 are, as that's the AC in. Do you have a link for the site it came from or can you take the cover off the box and see how it's actually connected?
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps