Tipper unit?

53 Reg Nissan Cabstar tipper, the tipper just stopped responding to the up/down control switch unit.
The switch unit has 3 wires the green wire has +12V and I assume that it is switched to 1 of the other 2 wires depending on which button is pressed.
When you press either button the +12V drops to 0v
When the main battery was flat the tipper still worked so does it have its own battery, if so where is it located?
Any ideas?