Sorry to drag up a really old thread but I'm having exactly the same problem and was wondering if you ever managed to fix it,
The HRW, cigarette lighter and horn are all on the same fuse, and are all not working, I have replaced the fuse and it doesn't affect it,
I was wondering if there was also a common relay that they all go through but from the haynes manual diagrams it doesn't appear so.
I've seen the link about my heaters but they are working fine.
Any help greatly appreciated.