If there are indeed two wires, didn't you disconnect the wrong one..? You would want the 'illumination' one connected, but the 'dimmer' one disconnected. The HU lights would always be on full regardless of the dash setting, but that's probably better than never on. The HU is low enough that it wouldn't be distracting if it was on full at night isn't it..?
(either that, or get Ian to come up with a dimmer conversion circuit..!
)