Can't get my head round what you've done here Sarah, I've had Mk4 dials in hundreds of pieces and they still work ok after. You changed the black facia from black to white? if so you'd have to had removed the needles as the facia won't come off without them being removed. Did you damage the needles removing them? if so the metal shaft on the guage could be moving but if you've broken the plastic needle shaft it will just spin inside the needle, so the needle won't move. Try removing the speedo housing again, it's a 5 min job, remove the glass, remove the 2 faulty needles, now refit the housing leaving the glass off, grip the petrol guage shaft lightly with your fingers and turn on the ignition, can you feel it move? if so you need new needles, I've got some spares around somewhere, red needles I think.
Let me know how that goes
Ken, October 2006