What is this switch next to gauges on Volvo 5.7

[Island Dreamer]

Took my boat out for the first time today! Cruised around the harbor and outside the breakwall to see what would happen.....I was shaking the boat down. Everything was pretty good. Still cant figure out how the Jabsco head is plumbed but the hand pump doesnt work anyway so I'm picking up a rebuild kit at West Marine tomorrow.
Could to figure out what this switch is to the right of the keys. I saw a voltage drop when I turned it on.

 

[tomc585]

blower?

 

[dcunniff]

That switch, goes up and down, lights up your gages at night.

Regardless of how often the head is used I end up rebuilding the hand pump. The weighted flapper fails and you can't just buy that part. It gets moved when you flip the lever for dry pump or wet pump out of the head. It fails wet pumping when it is bad. Good luck, the tightness of the small bolts may need tweaking to make everything work. And there is a "bridge" usually in the kit and it does not get used.