Just remember when testing the seats they only heat up when sat on.
wish I knew that yesterday, I fitted some 2008MY seats in a 2004 golf. I didn't bother extending the loom. I took the controller of the passenger seat with the loom and swopped it with the drivers seat loom. Wired it in and no heat. Checked the wiring with a multimeter and the voltage changed when the dial was adjusted. and doing a continuity test on the element that was fine. I was sure it was ok and put the seats back and when you pushed hard on the seat it started to warm.
Just noticed your reply (initially read this on Tapatalk and seemed to miss it).
It's good for the forum to know the box/loom swapping works. In hindsight I would use this method, but my seats were already in and I'm alright with a soldering gun & had lots of spare wire so didn't take long. Either way, it's still frustrating that Kufatec don't let you specify LHD or RHD as they are making the loom up to order anyway?
I know what you mean, when I ran it all in I had forgotten the 5A mini fuse so I was getting lumbar support but no heat - I was ready to pull it all out and start again until I borrowed a fuse and there it was - heat!
Big relief as I started to think the elements were gone, had I wrecked the loom etc.