If the boys at the bodyshop are any good with a soldering iron, get them to re-wire you a new ISO plug to the car.
Run a seperate feed from the battery with inline fuse and connect it to the yellow wire on the ISO, and also run a new ground wire to the black wire in the ISO (and mount it to a new ground point somewhere under the dash).
If you're using lots of power i would use this as an opportunity to upgrade the thickness of cable too
TBH, theres not alot in the wiring for it to go wrong. Theres only 4 wires (5 if you have power areal) that will ever be connected up on the power side, and 8 wires on the speaker side. If you're joining the red wire to the yellow wire, effectively you're only need to connect 3 wires to the power plug of the ISO.
ISO Power plug wires:
*Yellow = Continuous power feed
Red = Ignition power feed (disabled by joining with yellow wire)
*Black = Ground
Blue = Power Areal
Orange/White (IIRC) = Illumination (dont really need to connect up either)
So all you need to connect to get your headunit to work is the 2 wires with a * provided your red and yellow wires are spliced together
If its been metered and power is coming to the yellow terminal, check the join to the red wire. For your headunit to work with the engine off, the yellow and red wires MUST be joined together, and if the joint is dodgy etc, the headunit wont turn on as its expecting an ignition feed. By joining the 2 together, you're effectively supplying a constant ignition feed, so the headunit will work regardless of the cars ignition status.
Hope all thats clear
But in short... Double check that the yellow and red wires are both supplying power at the **headunit end**, and the ground isnt bad
