Power fuel pump using relay + ECM

Vehicle information: 2001.5 Dodge Ram 2500 Diesel 6 speed. The fuel lift pump is powered directly by the engine control module. The lift pump is powered while the engine is running. In certain cases it would be advantageous to be able to run the lift pump without running the vehicle. My thinking so far is to install a relay which has two switched power inputs. One input would use the existing power wire from the engine control module. The other input would come from a lighted switch inside the vehicle cab. Switching on either of these inputs would power the relay and send power to the lift pump (approximately 20amps of current) The switch wire coming from the ECM would be the only wire in the setup sensitive to voltage spikes. There would be no situation where both the in cab switch and ECM would both be supplying current to switch on the relay. Please direct me to a relay which may help me accomplish my wiring job.

Someone kindly directed me to a sort of FAQ about diodes. It looks like the easiest way for me to do this is to splice the wires together which will be triggering the relay to power the pump. I then need to solder diodes in upstream of the relay to not allow any current to run back to the ECM when the switch coming from the cab is used.