Battery position.

me maw's disco once used the handbrake cable as and earth path, it melted/welderd it solid and locked the handbrake on!!

Thats a similar thing that could have happened to me using the box as the earth, the handbrake is a good, well sort of earth connection if the strap fails.
A popular one with minis was that the choke and heater cables got the return current when the engine earth went, setting fire to the dash!.