Ok, I think I've worked this out but I had to take the unit apart to do it.
If anyone's interested the 3 unknown connections are as follows:
CON Ground (CON possibly should read 'COM')
B+ +12v Supply (Battery +?!)
V+ On/Off control

This AMP is of Chinese origin and a budget device at that, my guess is something was lost in the translation when they chose the panel markings