Here’s my boat wiring issue.
I got a 25hp Mercury two-stroke from my grandfather this weekend.
When I tried to attach the motor to the battery last night, I couldnt determine positive and negative battery cables as they are both black.
I would trace them back to their connection on the motor.
The positive will be connected to the starter solenoid and the negative will be connected to the engine block.
You can either do this visually or with an ohmmeter.
Hope this helps,