Not a Mopar But Ford Electrical Question

No lights or accessories other than the electrical Fuel Pump I installed (with an on off switch attached to it). The Battery works fine in my truck, but once i get the negative cable on (positive hooked up first) the voltage drops to zero volts on my multimeter set to DC Voltage with my electrodes hooked up the terminals themselves, not the clamps. Once I disconnect that negative terminal, the voltage on the battery slowly returns to 12.xx Volts.