I used 6g for Positive wire & no extra ground. Seems to work well with one battery although I did have major issues with how TM does their wiring which in a word was a JOKE!
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Billy
2008 3023
2016 Ford F150 Platinum FX4
2003 Land Rover Discovery
EAZLift 1000lb WDH & Sway Control
Tekonsha P-3 Trailer Brake Controller - Proportional