License Plate Lights not working

Hi there, I am wondering if anyone can help me out.
Problem is with the License Plate lights not working. One of the bulbs wasn't working so I replaced it with a new one still not working, the bulb holder looked corroded, I tried to clean some of the corrosion off but forgot to turn the lights off and it has shorted out and blew the fuse. (What a plonker I here you say, yes I am a 24 carat one I replaced the blown fuse number 4, but still nothing, I have checked the fuse holder with a volt multimeter (Lights on) and am only getting 0.4v and should be getting 12v. So I assume its taken something else out when the fuse was blown. Can someone point me in the right direction? All the other lights etc are working fine. After reading the forum, it may be the light control unit, light switch or a relay but unsure and only assuming.
Thanks for any help in advance.