Secondary fuse box location

Hi All
I have a '98 Defender 90 TDI, according to the handbook the secondary fuse box should be under the drivers seat. The fuel tank is there, and no sign of a fuse box, I have checked under the passenger seat and everywhere else I can think of. My problem is the headlights do not come on and main beam will come on with the flasher but it will not stay on. The main switch is working and I have checked the fuses in the main fuse box. I suspect that there is another fuse somewhere but I have been unable to find it. Any ideas?

The main beam / dip switch may well be the problem. The underseat fuse box only came in when the tank went to the back.

Thanks,
so you are saying that there is no secondary fuse?
The switch appears to be working - I get a reading on my meter when I switch on. It is difficult to trace the wires but I think they go into a relay, could that be the problem?