Some of the latest Canbus systems don't feed 12v to the reverse lights... They only feed around 3v.
Hence Canbus parking sensor systems needing a ignition 12v relay wiring in...
As normal kits need a minimum of 9v to work...

I'm having the same dilemma with a 62 plate Octavia, but I've just bitten the bullet and ordered a plain £9.99 reversing sensor kit...
Worst comes it's no good for his car and we need to buy another kit.

I'm pretty sure a Focus will be fine with a poverty kit...