Could be an immobiliser issue with ignition and no crank!
Also what does the red light by the gear lever show and the dash display show for gear position?
On my L Series when the car's locked there's a light that flashes on the dash. If the immobiliser kicks in - I'm sure this stays lit - showing that the immobilier will not let the engine start.
Presumably the light you referred to above does the same on the V6?
Given the amout of work you've done on your TD4, I'm sure you'll find the answer to this 'reasonably easily' (famous last words

).
Going from that other thread (which was a K Series)...
The starter solenoid takes its power from fuse F1 in the engine bay. It is enabled by the starter motor relay.
So, wire to from + terminal on battery to fusebox in the engine compartment, fuse f1, wire from fuse f1 to starter relay and finally wire from relay to starter solenoid. The solenoid is earthed through the engine.
The relay is energised by a positive feed from the 'crank' position on the ignition switch - but its earth goes through the CCU - so to be energised the key must be on crank and the CCU must ground the earth (if the immobiliser's on it won't do that).
The wiring is a bit more complicated. Same wire from the battery to the fusebox in the engine compartment. It looks like power goes through fusable link 4 then fusable link 3. From FL3 there's a wire to the ignition switch. From the 'crank' position on the switch theres a wire to Fuse 5 in the passenger compartment fusebox. From Fuse 5 theres a wire to the starter motor relay. From the relay there's a wire to the CCU - and from there I should think it has a common ground for various purposes.
As I said this is the layout for the K Series. The KV6 will probably be different - but worth checking the immobiliser then the fuses, which ever they are on the KV6 (Rave will be your friend), relay and wiring.
On the other thread, it is because of a fuse that keeps blowing - I don't think its anything to do with the starter wiring (although it might), I think its because the fuse powers an O2 sensor relay as well and I think that wiring is blowing the fuse. The OP hasn't been able to confirm yet.