I had a 1998 Volvo S40 (non-automatic) and fitted a Parking Partner reversing sensor kit, taking power by tapping into the reverse lamp lead, and it worked perfectly.
I now have a 2003 Volvo S40 automatic. The body is essentially the same but the electrics are more sophisticated and less accessible. I asked my local (non-Volvo) auto engineers to fit the same kit but it repeatedly fired off erratically when nothing was there to trigger the sensors. This persisted in spite of trying different earthing points and fitting a diode on the 'power' side of the kit (as suggested by the kit supplier).
The local lads substituted a different make of reversing aid and it does the same (but worse!). Before trying to run it from an alternative 12V DC source (to be switched on/off via a reIay on the reverse light circuit), we have tried some logical experiments.
The guys told me that it worked OK when powered by a stand-alone 12V DC battery.
But when powered from the vehicle's built-in 12V DC cigar lighter connector it starts to malfunction (unpredictably) as soon as there is any change in electrical function e.g. if the footbrake is applied, the windows lowered or whenever the engine is running. It even fires off spotaneously every 8 seconds when there is nothing obvious happening to account for a voltage change!
Apart from using a (non-autochargeable) 12V battery pack and replacing it when it goes flat, I can't think how to resolve this - can you?
Lionel