Looking for D+ Ignition line for BCDC Charger detection of Smart Alternator

[KirraweeBob]

I am installing a Redarc BCDC1250D charger in my caravan pulled by my 200 series. The instal manual says connect particular wire to D+ for Smart Alternators. Question is how can I locate a D+ ignition line? Is the re one in a 12 pin trailer connection?