I've got a car dvr system that I replaced because it was a little unreliable (turns out the battery was duff, so it'd just power off mid-save and corrupt the sd card). It's not *technically* waterproof, however, I've siliconed the inside of the cameras (they're not directly being sprayed/exposed to water) and I'm going to see how long it lasts!
Front camera is mounted inside the screen, and rear camera (if I fit it), will mount just to the side of the reg plate. I do also use a Ghost X helmet mounted camera, but once it's battery has run down, that's it, where the bike mounted one will just loop record.