The fuel harness runs under the driver seat, when the gauge has no connection the will read full.
My truck the harness had frayed on the seat bold and shorted my fuel pump to the fuel sender. I pulled the harness out of the truck and repaired it with solder and heat shrink. It wasn't a complicated process, but the bed and tank needs to come out/off. No if the connecter is shot then the dealer has parts for it, you can replace it.
So my advice is to crawl under your truck and check it out. Look where the seat bolts for the driver seat come through the floor and these harnesses sit on top of a plastic shelf on the inside of the frame.
99 x-cab 4x4 4.0l auto
mods, cranked t-bars, silencer delete, k&n drop in, Rebuilt trans ,Trans Go stage 3 shift kit, 3200stall Fuddle Racing Torque Converter,
VANIR 63mm TB, Roush split rear exit exhuast, E-Fan, JBA Headers
Auto up/down both windows, OHC, EATC (soon to be modded)
03 Crown Victoria Police Interceptor