You can peel the rubber cover off the switch and get an eyeball on it. Maybe the switch is just dirty, try blowing some air through it. And maybe a small drop or two of silicone spray on the end of a toothpick to lube it.
If the switch is bad, it'll have to be soldered out and back in. There's a pile of vids on YouTube if you're handy with an iron. It's not an uncommon repair, so wouldn't be surprised if it was bad.
I poked around a bit, but don't see where anyone has found these switches outside of Garrett. I'd be happy to repair them for you guys if I could source the part.