I guess thats it in the end.
Its what the engineieer wants, that what they get.

For me, its a safety issue in that you have a high amperage cable running though your cab without any protection (fues, breaker or relay). Sure its probally never gonna be an issue, but that then becomes a sticky issue about 'what if' senarios that become difficult to accuratly quantify.Analyse the risk associated with it, and you find that you need to do something about it. I could rabbit on for a while about this, but I think you and I are arguing on the same side basically Bailz.

Anyways, the original question of where to run the cable was offically answered by the engineer. Outside.

As to cable size, bigger is always better, and 19mm (dunno CSA size) diamater cable will be more than enough.