I would use a new earth and live, straight from the battery, the chassis where it is exposed is going to build up a high resistance.
14/030 wire is more than suffecient for this, use the live after the cut out, so keeping that in circuit.
If you look behind the rear seat on the metalwork near the top there is a 8mm hole that is just right for securing a relay holder to, you can then take the relay out as an added security measure, if at shows etc.
tabetha
tabetha