How do I know wich one is the relay for the gaz pomp on a 1999 Mazda B2500

You look it up in factory service info , look at a wiring diagram , fuse box diagram etc.... This vehicle also has an inertia fuel shut off switch . You can view all this at http://www.bbbind.com/free_tsb.html

Also know that the PCM - engine computer turns on the fuel pump relay . If there is no crankshaft position sensor (RPM signal ) to the PCM ,the PCM will not turn on the fuel pump relay . Relay #5 is fuel pump relay . Do you know how to test electrical circuits ? Use a Volt meter ? Or are you just going to replace the relay and hope that's the problem .