Newbie questions on my 2010 Cali

I have a Cali from the same year. Picture one there is the tender which is kept inside the red bag and normally stored in the trunk.
Picture two is a view of the trunk and the arrow points to the little kind of flap door where is hidden the female plug where you connect the male plug of the tender. This little door as a lock which you open rotating using a coin or screw driver. On mine when I'm not driving the tender is always plugged and that way the battery is always fully charged and the probability of appearing electrical or electronic issues is much lower.

Mine hasn't the switch shown in your picture so it doesn't look to be standard and better ask the seller what's it for.
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The switch and knob is likely the on/off switch and volume control for a K40 radar detector, which may or may not still be in the car.

 

The K40 usually just has a pair of LEDs installed somewhere, if that's what it is. Might have a remote control. It's not a terribly well regarded system, or at least it was not in the past. Might be better now. Often installed at the dealership. I've ripped a few of those out by the roots in various Porsches over the years. See if there are any extra "lenses" in the front grill and back by the rear license plate, that would indicate it had laser jammers. The radar antennas are black boxes. Probably a speaker somewhere too. It probably beeps at you and the LEDs should flash on startup, if it's still intact.

You can see some of the pieces visible on a California in the second post down ... this is a "hall of shame" site showing botched installs.

https://www.gmpperformance.com/index.cfm?pg=carGallery&mode=displayVehicle&VID=3808#!prettyPhoto