Ahh, well, now that's a good question. I have done it differently this time. I'll stick pics up of all the work when I next do Cruiser of the Month
I decided that I didn't need the rear heater matrix, so I took that out so that I could put my GPS brain-box under the seat on a little plinth. This leaves a nice hole where the heater pipes go through. So, I have come from the battery, down the firewall, through the holes in the chassis legs and up through the hole into the cabin. The straight to the sill and under the carpet along the scuff plates over the wheel arch, behind the plastic panel and he presto, out to the rear battery via the Cyrix which is mounted, again, in the subwoofer hole. Bulkhead has a joint in it with a Mega fuse and also a master off battery switch.
Cable, I'm paying £3.99 m plus vat. Very flexible as it's welding cable, no issue with amps that's for sure and of course the sheathing is pretty durable and heat / spark resistant. 50mm2 as I said earlier.
All gone swimmingly well so far. Should be a 'clean' day tomorrow. Exhaust all checks out, no leaks. Incidentally, I now have a Toyota OEM exhaust system for sale. Pretty sure it's genuine. All stamped by Mr T. Very little corrosion and joints/welds all look good. There is thousands of miles left in it. I shall stick it on the for sale section. Might be just what someone needs.
Is that all? Well I'd have got more done if it wasn't for the constant interruptions from people asking how it's going!!

Only kidding.
Chris