If you're talking about the part that screws into the driver's head (of the car), they don't just "quit working". What quits working is a break in the electrical line, or whatever it connected to, like a gauge.
How hard to gain access to the gauge?? I'll tell ya what "I" would do is run a separate wire thru the firewall and connect to the gauge. Cheaper than a new gauge. If that doesn't work, I'd get a new sender (cheaper), but I don't really think it's a sender issue. I'm ASSUMING you're getting NO reading on the gauge. A sender that's going bad will give a bad reading, but noat a NO reading. That's usually a burned up gauge.