Buck converter extreme voltage drop?
If you are loading a 12V->5V step-down converter with a 2 A load (fully loaded/running PRi3, or 2.5 Ohm resistor), you should expect the input current to the convertor (on 12-V side) to be about 1 A (assuming 80% efficiency).

A correctly wired car cigarette lighter outlet can deliver 10 to 20 A of current at battery level (~12 - ~13 V). So loading a lighter outlet with 1 A should result in no visible drop in voltage.

If you are using some non-standard hook-up to car battery network, and experience horrible voltage drop, it only means that you somehow got the power incorrectly. Check your car schematics for right wires to hook.