Mercury Classic 40 hp Oil Injected ground wire fried?

[Hitman73]

My brother in law has a Alumacraft with a 40hp mercury classic oil injected outboard motor. He came home from fishing and noticed some wires had been fried and melted. He said it started right before they got in and loaded it for home it was smoking a little bit under the motor cover. When he took off the cover and inspected the wires the ground wire had fried right in half and no longer connected.

The ground wire fried all the way to the wiring harness plug connecter and melted some other wires. What could have caused this to happen?. The rest of the wiring looks to be ok going out of the motor, just the ground wire has fried and melted the tape and some other wires on the wiring harness all the way to the plug inside the motor?.

Any help much appreciated!!!

[Hitman73]

Added additional info!>

Motor-
Mercury 40hp Classic, Oil Injected.
Serial#- OD210696 or 0D210696

Any idea what year the motor is?.