Just spent 2 days installing a light bar to my Scudo. Most was wasted time as despite all care and testing I couldn't get it to work. I used the existing wiring as I assumed it was installed correctly.I tested every individual wire and connection with a multi metre and everything was correct , except that the light didn't work. Checked the relay it worked , checked that voltage across the light connection which read 13Volts. Checked the Forum and online but had no luck on what the problem may be.I was ready to give up

Eventually I disconnected the Earth connection bolt in the engine bay ,cleaned the terminals and put everything back as it was. That's all that was necessary. Everything works perfectly.
It seems that with LED lights, a perfect electrical connection is required. The 13volts obviously just wasn't adequate for LEDs.
Advice to the uninitiated

Did you pull cables through the firewall or did you find a power source in the engine bay?

All wiring was in the engine bay, but I did need to bring over-ride switch wires through the firewall. That was easy enough , I just didn't expect that perfect connection was necessary for the LEDs.