Wiring up 3 alternative power supplies for a new system
I understand that your equipment is to be powered by an internal 12V battery / internal 12V power supply and occasionally by a 12V vehicle battery.

A Schottky diode would ensure that the power supply and the vehicle battery, with a slightly higher voltage, would override the internal battery.



A DC barrel jack socket (with battery disconnect) would be required on the equipment to input power from the vehicle battery and, at the same time, disconnect the negative line from the internal battery and power supply. 
A cable, with a cigarette lighter plug at one end and a DC barrel jack plug at the other, would also be required for the purpose.