How to install a wiring harness in the tailgate of a VW UP?

Hello,

I had to buy a new tailgate for my VW UP. Unfortunately, I got a tailgate where the wiring harness had been cut off at the ends.

I still have my old tailgate and it has a fully intact wiring harness.

My question is, how do I manage to install a wiring harness in the tailgate? Unfortunately, it is not exposed when you remove the plastic trim, but is enclosed in a metal frame.

I'm grateful for any help and if anyone knows of a video that shows this, I would appreciate it if you would post the link.

Kind regards