So you are still using the harness wires that ran from the switches to the motors? How large is that wire? Maybe you should have considered (reversible) disconnecting the original harness wires and running larger motor wires from the relays to the motors. Relays are not just to save switches. In a "clean" design, you would mount the relays at the load, or close to it, so the undersized switching wires, as well as the switch, are relieved. But it's also the MOTORS. It generally is not good for motors to be run at too low voltages. If nothing else, that increases the damage to brushes and commutators