You can remake the harness, it might not save you much money vs the genuine Helix T Harness. You'll need ~ 30ft of 9 conductor speedwire. And a quadlock extension, preferably with the removable blocks. Will be hacking it and splicing it for the correct pins and plug and play aspect.
The VW Helix amp unit, uses a regular 20 pin molex connector, which you already have.
You'll need two runs of 9 conductor speedwire. 8 for the inputs and 8 for the outputs. There is a 9th remote wire in the speedwire bundle, but you're not using it.
Then you have the option of hacking up the extension harness and splicing in your wires from the amp. I couldn't make it fit with the quadlock extension and the OEM wiring.
I have a different setup for power and ground running straight from battery. But this is the pins from the extension harness you'll splice and teeing in.