You should have 2 wires down the tower, positive from the rectifiers goes to positive on the battery. Similarly negative from the rectifiers goes to negative on the battery.
There is no circuit to the heat sinks or any of the machine frame.
It is normally a good idea to ground the negative lead and also ground the tower for lightning protection but this is not needed for the thing to function.
In most cases it may be best to ground rectifier negative and tower directly at the bottom of the tower. If you are in an area prone to lightning there may be other considerations about where you ground and it could be influenced by such factors as having grid connection and other grounds. Too complicated to give a general answer about lightning protection, but the dc path is completed by the 2 leads down the tower.
Flux