Rear registration light doesn't work ...have changed bulbs and fuses still not work...

Could be just a bad connection. Check it for voltage with your lights on. If you get 12V, your connection is probably either weak or oxidized. You could try bending the connector wires out a little if it's a plug-in type bulb, or cleaning the socket if it is a socket type bulb. It helps to use some dielectric grease on the bulb/receptacle as a preventative measure against corrosion.