FoCo_4_Low

Hi guys!
I'm still trying to track down an electrical short in my coolant temp. sensor circuit. The wiring diagram in my manual shows that the sensor has two wires, one running to the ECU and one running to the vapor pressure sensor. After doing some research I am confused on what the vapor pressure sensor is and where it is located on the truck. My manual shows that it is mounted to the firewall above the brake master cylinder, I can't find anything there. Some sources indicate that there is a vapor pressure sensor in the fuel tank. Does anyone know what this is and where it is located on a '95, 3.4L, 4x4.