I had to replace a complete door mirror on my x244 last year. Access to the mirror connection was behind the door speaker and carefull paring away of the speaker shroud was required.
If your x290 is similar, I advise you to investigate and think carefully before proceeding.
As regards power supply, the mirror demisting supplies are one possibility. Does your x290 have side marker lights as distinct from turn signals on the mirrors. The supply to any such lights would present another possibility, but there could be CANBUS compatability problems.
Thanks for the suggestions Communicator.
I was really thinking of a power source to switch the lights on when the door is opened and off when the door closes. I managed to achieve this on a VW Camper I had, but don't know where to start with a Fiat...
You probably have to run a relay from the door switch so it switch's on the power feed when it's unlocked then run a second power feed into the door for the lights to run off
It could be worth downloading eLearn for the x250, in the hope that the wiring is similar.
On my non canbus x244, there is a 6 way connector, D042, for the courtesy lights behind the RHS fuse panel. Wire colour for the light operation is black/lght blue, connected at pin 4.