Power supply advice in 480, please.

[blue and cream]

I would like to have a permanent power supply for my Garmin SatNav, rather, than plugging /unplugging into the cigarette lighter.
I tend to rely on the Satnav for a speed reading rather than the temperamental and unreliable speedometer: over the years the garage has fitted a new circuit board, a new speedo head, and a new speed sensor. There are still problems. Otherwise I love the car, a 480 Celebration Auto.
The Satnav normally sits in the recess shelf in front of the speedo, and it seems easy to take the cable from the back of the Satnav through the gap where the horizontal shelf meets the bottom of the instrument panel glass. But where can I take a power supply from? I would prefer the source to be ignition-controlled, to eliminate leaving the Satnav switched on and draining the battery.

Any help would be appreciated.

Brian