LEDs have a driving circuit to them, putting a potentiometer in there will just under power the driver and probably do nothing to the LEDs brightness at all. In order to dim LEDs, a driving circuit turns them on and off really fast with a PWM signal, not variable voltage. I'd recommend trying to build a LED driving circuit with an Arduino to get a mechanical understanding of how they work, then never touching your headlights. Proper headlights are lensed so they can be bright but not shine into people's eyes. If your Prius does not have LED lights from factory, but you want brighter lights, just replace your current halogens with higher quality/brighter halogens rather than poorly implemented import LED bulbs that'll rarely work right in halogen reflectors and cause you to be an on road nuisance. You are probably also due for a headlight polishing, which you can buy a kit for, and will significantly improve your headlight brightness.
