Horn + Front/ Rear Cig Lighter Not Working

[Stuie]

Hi All

The horn and both front and rear cigarette lighters on my 2003 ML270Cdi (w163) have died. Checked the fuses, all ok. That's where my limited electrical knowledge ends i'm afraid. I don't smoke, but my daughter uses the rear socket for a dvd so shes not a happy camper. Strangely, the 12v power socket under the glove box is ok.
Has anyone experienced this and sorted it. I'm aware if i dont get the horn going it won't pass MOT. Bit of a bummer really.

Ta

 

[BlackC55]

Where did you check the fuses?

 

[Stuie]

Where did you check the fuses?

Under the bonnet.

 

[Stuie]

Ok. I checked the fuses in the drivers footwell. The fuse for the sockets had gone so I've changed that. Alls well there.
However there isn't another fuse for the horn there. Only under the bonnet. That fuse is fine.
Could it be the horn relay??