Ask the replacement harness vendor about wire size. I bought my 3600 with a replacement harness already installed. It looked fine. However, the truck is still (will remain 6 volt) and I had low voltage to the headlights and heater blower motor causing dim lights and a slow, low air flow heater motor.
After searching for bad connections and grounds I resorted to using relays to fix the problems above. All's great now.
The replacement harness had 18ga wire going to the headlights which I believe is undersized for a 6v system, likely OK if 12v. I didn't check the wire size to the heater motor but it was Much smaller than the original it connected too at the motor.
Good Luck,
Ron
