John
Mine's 1996 with no dim-dip, so the wiring may be different, but this may help:
My headlight fuses & relays are in a box in the engine compartment behind the LHS battery. The cover is clearly marked with separate fuses for left & right / main & dip (ie 4 fuses in all); also the main and dipped relays are marked. If your cover isn't marked have a look in the handbook. If you don't have either I'll sketch out the layout for you.
(1) Obviously check the fuses first, but if all main beams died simultaneously then I don't think this will be the problem as it would require two fuses to fail at the same time.
(2) Next turn on the main beams, and have a good fiddle with the main beam relay. It's possible that it is poorly seated, and you may find that you can persuade the lights to come on. If this is the case it is not necessarily "problem solved": pull out the relay and check for signs of sparking / heating on its connectors. If there is any carbon, charring or discolouration then this indicates that the connection was poor, and it may have weakened the socket into which the relay plugs. You may be able to crimp the socket more tightly with long-nosed pliers, but this can be a bit of a fiddle and there is no guarantee of a long-term fix.
If that doesn't work try swapping over the main and dipped beam relays in that box. (I haven't tried this, and on mine they are different colours - presumeably because the high beam carries more load, but I should think they are interchangeable for testing purposes.) If that now works, and dipped beam doesn't work, you've found the problem: your main beam relay has died.
I recall that you fitted higher wattage bulbs recently, so I think this is the most likely cause. If the failure was due either to the relay itself, or its connectors, you should consider fitting some sort of relief wiring to take the heavy currents out of that circuit - and your lights will be brighter too!
(3) If that was not the problem then I would suspect the steering column switch. Try pulling it towards you in "flash" mode - if that works while conventional main beam position doesn't, then this is the problem. (But this is is not a definitive test to eliminate that switch).
(4) Another possibility is that one of the fusible links (from the RHS battery) supplies the main beam circuit. I don't know whether this is the case, and I haven't traced that wiring on mine. If we reach this stage I'll do some more tracing for you, but I'm fairly confident that it will turn out to be the main beam relay.
Christopher Bell
Devon, UK
1996 1HD-FT
Seems to be that kind of day problems and all. Well heres mine imstill with the misses, na only messing. I was out untill now collecting the young fella from his friends house about 15 miles away , on the way back on these lovely bendy roads on full beam, all went black. Well I wont say what was said but instinct took over and I went to dipped . The problem is all the four head lights just went off altogether, are gone all the head beams, WHY. As usual I dont know why I have dipped and no head beam in all four .