Update time:
1: Installed new wiring in the boot lid that runs back through the hinges and into the car
https://www.ipdusa.com/products/5964/102560-volvo-tailgate-wiring-harness-right-mtc-vr333-1348019https://www.ipdusa.com/products/5966/102561-volvo-tailgate-wiring-harness-left-mtc-vr332-3544695
This has resolved the intermittent wiper issue and the home/park position.
However the demister still doesn't work, here are some figures:
13.3V when I check voltages between the terminals on either side of the grid.I'm getting continuity and have 6.55 Ω at the terminals, if i check the Ohms further upstream within the bootlid it's 6.51Ω
I know there is a couple of stripes broken and was considering using this kit to see whether it effects the system operation:
https://www.permatex.com/products/specialized-maintenance-repair/defogger-repair/permatex-complete-rear-windowdefogger-repair-kit/