No measurements, I gauged the two end ones from web photos, then made the three spaces equal. (Use loads of masking tape)
No, I have an Xtrons head unit that has a optional reversing camera (which I also fitted), which happens to have a live for the reversing sensor.
Each of the sensor cables plugged into a small junction box in the rear, and a much longer cable from the rear juction box to the sensors head unit on the top of the dash.
IIRC the cable from the dash readout, plugs into to the juction box in the rear, and the juction box needs to be powered. I took that power from the Xtrons head unit because:
1). The cable was supplied in the kit.
2). I know the lighting currents are monitored by the ECU's and didn't know what current the retro sensors consumed.
I'd have to say no problems and straightforward, but I'm an electronics engineer so I would say that wouldn't I.