Depth finder sometimes comes on and when I really need it, it doesn't

[Soul Mate II]

Looking for some help with where to start - I have an older depthfinder but it works as it should when it comes on. When I turn it on at the dash sometimes it comes on, sometimes it doesn't. With that being said, it isn't the fuse - do I check and clean the connections?

Should I take the dash out and clean all the connections behind the switch?

Where do I start?