HID Kit wiring harness necessity?

I bought the 5000K H11 kit with slim ballst. No wiring harness. everything was just plug in. took about 30 minutes to isntall.

 

The question I would ask is "why not use the harness?" Sure, it's a little extra work to install and dress properly, but in the end, if *anything* goes wrong on the wiring end, it's all in a place where you can get to it and replace or repair without having to investigate and extract factory wire runs. It's cheap insurance.

Another thing to consider is that the power supply characteristics of this particular ballast are likely not documented or thoroughly known. It is entirely possible a current spike of a sufficiently short duration could fail to blow a fuse, yet still damage a semiconductor. It's a little more of a long shot, and I don't know enough about the Prius' headlight circuit to know if that's a danger or not, but again - why take the chance?

To answer your question, the terminal i've seen people use for positive supply is in the fuse box under a small cover. You'll install a ring terminal on top of the existing lug. Ground can really come from any metal you can bolt to, but I've personally seen a dedicated ground terminal on the drivers side just outboard of the headlight, about bumper level. The advantage of using that location is that you can minimize the ground potentials between your ballasts and the true ground. Ground loops are ugly things.