The plug on the left is the factory sub connection from the car, and the plug on the right with the yellow wires is what I made with the VW parts noted in the first post. This allows you to use the factory plug and quickly disconnect or pop back in the Fender sub as needed.
For wiring, quoting the Vortex thread, "the two white wires are positives and the brown wires are negatives. There is a yellow stripe on one of each color to distinguish which belong together as a pair."
PRO TIP (that I didn't know): Home Depot sells 16-gauge speaker wire by the foot in the electrical section. When you're picking up your MDF or plywood, grab 3' for inside the box to connect the speaker to the terminals. It was .52/ft at my local HD.
This is the sub I used:
https://www.sonicelectronix.com/item_36297_Rockford-Fosgate-Prime-R2SD2-10.html