Starting problem om my 2,0 tfsi quattro 2005

It suddenly wouldn't start. Starter cranks the engine, but nothing else happens. I haven't been able to hear the pump working, but I'm not sure if it supposed to start priming when I open the door or if I have to turn on the ignition. I have tried to feed it with 12v directly, and it seems to work fine then. Next item to check i guess would be the fuel pump relay, but I haven't managed to find it yet. I have tried to find info about it online, about where it's located and what it's called, but I get tons of different opinions. I went to a Audi dealer and bought a relay, and got a "646". It got me confused, because although I had found different info online, no one had even mentioned relay 646? The guy said it had been changed. Now I hope I can find info here about where I could find the relay, and what it is supposed to be printed on it?