I don't know how complete it is, but it seems to list cars, billboards, and speed traps. I think people are still updating it? If I come across anything better or different I'll be sure to let you know.
Edit: Oh it looks like you can filter what you're looking for. Just change the filter to Jackspots. I still don't think it's finished. But if you keep an eye on it, I'm sure it will be soon. I know there was an interactive map on IGN for Forza Horizon that was filled out REALLY fast.
Its located at the very bottom and to the east. Turn off the music in game and you can hear new cars playing their stereos. Its through a tiny security fence on the south side of the street. --- 2500k @ 4.2 | P8Z68-V Pro | H80 | 8GB | 670 | 256 ssd | 6Tb hdd | Win 7 64bit | ax1200w | BD burner | cm690II Steam: DV8ing1