Finding a D+ connection

Hi,
I think my new split-charging setup needs a connection to "D+" to energise the alternator... I haven't been able to find a wiring diagram for my "3L" model, can anyone advise what to look for or where I might find a diagram to help me identify it, please?

The setup worked OK for a while but it seems it can get into a state where the alternator needs an energising connection in order to kick in- there's a terminal on the charger that is present for exactly this purpose but I need to work out what to connect it to!!!

Thanks for any help :-)

Cheers,
W.