Fitting dual batteries in GU Patrol

u mean in the cabin or under the bonnet?

By the "switch box" I assume you mean the Isolator Unit?

Normal place is on the rear firewall in the Engine bay, central to the location of both batteries.

A dry place like the drivers side of the brake booster below and to the drivers side of the wiper motor is pretty good, remember that anywhere on the passengers side of the motor gets Quite hot from the exhaust manifold and turbo.

Hope this helps,