A couple of weeks ago I brought some tools down from MI with my wife's 4runner in a U-Haul trailer. The stock wiring harness was not working properly since I was not getting any daytime running lights. My dad would not let me leave until it got fixed. I left for a while and he decided to fix it for me. Well to make a long story short the gas gauge is on the frizt. Sometimes it will read full, even if it is not, and then after driving a little bit the gas gauge will plummet and the light will blink telling me the tank is empty even though there is sometimes a full tank. I disconnected his Amish trailer harness wiring so what else could be causing this?

 

Anybody out there have any input?