You might be able to tell by looking through the front of the light to see if it's blown. Sometimes it's obvious, sometimes you can just see one of the filaments (fine little wires) is not contacting both ends where it should be to create the circuit. Sometimes with the lights on you can give them a hit and the filament will recontact and start working all of a sudden.
I've presumed bigger problems before due to two lights being out only to find out after wasting time it was just two globes gone!
It seems unusual that everything else works, just not two (of 4)headlights.
Start with the basics first- globes, power then earth.

I haven't done anything to the car. It's a one owner that I started actually driving so some of the gremlins came out.

What I know...

I have no clue on electrics
The lights were working fine.
Both high and low beams don't work.
Driving lights and tail lights do.
One of the four wires coming out of the relay box blew (literally caught fire) and I replaced it on a trip to Goulburn and I have since replaced all four with new 15 amp copper wire but using the old terminals.
I have a feeling when they blew six months ago one headlight on low beam was temperamental before I replaced the one relay wire and when they both went I thought it might be switch related.
I haven't changed any thing from how it was wired.

Shoota I don't even know how to do that... What are the chances of them both going? Can I do a visual check???

With that advice IM,turned on.


He has 4 lights,but same principle. IM more,inclined an earth or try the head,light relay

Quite often Datsun switches can develop corrosion inside the switch over time so it's defintely worth dismantling it and using fine sandpaper to clean the terminals.
Please don't be offended by this question but have you put 12 volts directly to the two lights in question to make sure they haven't both blown in tandem?
Try that first and if that's fine pull the connector off your light switch and make sure you've got 12 volts at the switch. Then plug it back in and make sure you have 12 volts coming out when the lights are in the on position.

Some of the wires in the relay were pretty buggered and I thought I'd found the problem, so I re-wired them but still have the problem. SO I assume now it is either the earth from the relay(?) or the switch itself.

How would I go about checking the earth?
I'd like to do this first as getting to the switch is difficult, it looks brand new, and the plastic surrounds around it are brittle.

It is not a common problem for the headlight switch to go bad. Far more likely to be a connection problem, or the Headlight Relay. However at 40 years old, anything can happen. Since Datsun 1200 does not use a headlight relay, the switch is under more stress than a 260C.

This happened to me I changed the switch and it fixed all my problems

I had a similar problem on my 120y, and found out that the main wiring harness under the dash where it connects to the engine harness had a dirty corrorded pin, pulled the connector apart and cleaned the pin and problem was fixed.

Could be switch but last time I say in a 260 was a taxi in Singapore

Stock.
Where would I look for the ground?? I have a feeling it might be the switch itself. Do these commonly go??? And I can't seem to remove it easily.