Hi Eddie, if it were me I would purchase the camera and try it out on a very temporary lash up with the usually quite long supplied lead with cigar lighter plug and plug it into your 12v socket, trying it out during daylight, dusk and night, then check the results. Halfords have a good returns policy and should refund if not suitable.if the darker lighting situations are not good enough quality then you may have to pay for a more expensive model.
As regards a permanent setup if it goes ok then I'm unsure about the saloon CC but I reckon you could get cable down the back of the seats, under the boot lining, to the 12v socket in the boot. You would need to tap into this with a fused cable. This would be ideal as the power here will be ignition switched live.
Good luck with your project, it's a shame these are not factory fitted or at least an option!