I have a 1999 C220 CDI estate Sport that has 3 problems I cannot get to to the bottom of. The heater booster and heated rear window and horn will not function. The light on the rear window heater lights up but a continuity check at the back screen shows no voltage is getting through?

The light on the CDI heater booster too lights up but it fails to function?

The horn does not work yet when the alarm goes of the will sound fine?

I have checked all the fuses and all are well in the front and rear fuse box. I have cleaned up the earth over ther left hand wheel arch and it has made no difference. What I am hoping is that there is a simple common link maybe a relay or something that would link all these problems together. Any ideas would be greatly appreciated... Many thanks.....