gas gauge going crazy

For about a year now my gas guage will bounce from empty to full back to empty then maybe only half way etc. Very erratic. When tank is completly full it does not happen, but when it's down to about 3/4 it starts. I have always thought it might be quite expensive to fix so I just watch my milage to know when to fill up, but have seen suggestions on grounding issues. Where are these grounds located. I've got a 2000 s10 blazer. I just replaced the fuel filter and ran valvoline fuel injector cleaner through it, still no luck. Runs a lot smoother now, no rough idles, but still non-functional gauge. Any help would be appreciated.

The ground for the tank (pump and float) is not at the tank. Trace the wire loom from the top of the tank to the frame rail and eventually, you will find where the ground connects. Clean it and reattach. Fuel injector cleaner would have no effect on the float.