Ignore them two

It's an urban myth that they don't work. Mine works perfectly......
You can access the wiring by wriggling under but probably easier to pull of the dash plate.
Sender unit inside the tank is notoriously bad. More likely to be that. The float has a habit of becoming holed, fills up with fuel and sinks to the bottom.
Also check your earth from the sender is good. If your water temp gauge works then your fuel one is probably ok, they share the same power, wired in series.