They come with little metal tabs. The license plate bolt goes right through and they just mount right up beside your plate.
http://www.lights.skenedesign.com/P3_Installation.shtml
http://www.lights.skenedesign.com/Brackets.shtml
http://www.lights.skenedesign.com/P3_Gallery.shtml
The PCX is not listed since it doesn't require a proprietary mount. The universal tabs work great.
The nice thing about the Skene lights is that you can program them different ways. If you don't want them to flicker, you can easily program it out in 5 seconds. Same with the brake light flash. You can customize the tail light effect and brake light effect separately.
If you want turn signal function, you'd want the P3TS model. Or just the P3 without signals.
Here are the programming modes:
http://www.lights.skenedesign.com/P3_Programming.shtml
I just went down to the garage a few minutes ago and turned off the running light modulation and changed the brake light flash. It's legal to flash for 5 seconds in Missouri. All the flashing modes revert to steady state in less than 5 seconds. But you could run a straight up standard brake light if you wanted no flash at all.
I've had Hyperlites before as well, and would recommend them as well. I just got a deal on the Skenes with promo pricing at the BMW rally, so I bought those.
I drilled a tiny hole under the license plate, ran the wires through the fender there, and then up through the small holes where the turn signal wires go. Then wrapped the controller in bubble wrap so it wouldn't rattle around, stuffed it all in the tail section dead space. The Skene kit comes with super easy to use wire taps. Easy. The main problem is getting your hands in the tiny space back there to get to the correct wires. Pop the tail section of the plastic off carefully from the front first.