My defender thinks it's a police car!

The headlights on my 1996 Defender 300TDI have taken to flashing on and off whenever the ignition is switched on. Disconnect the connection to the hazard warning light switch and it stops - but I then loose indicators and headlights. Main beam also seems to have stopped working. There's obviously a short circuit that's routing through the flasher unit (I've confirmed by feeling it operating when it's doing this) but where do I start to look? Oh - and the trailer indicator warning light on the dashboard is indicating at the same time. I'm pouring over wiring diagrams trying to figure out where to start looking but thought I'd call for help on here as well. Any thoughts people?

Firstly Welcome .

Do you have a DMM (multimeter)? Cos you are gonna need 1.
My first guess would be some dodgey earth/s somewhere.
Also check your fuse box hasn’t had a burn on a fuse.

My trailer light flashes too, yep it’s got a little fault somewhere, when I have time I will look but it don’t affect anything else, yet.

But best guess something is earthing through your indicator/hazards.
Pull the hazards relay?
Pull the indicator relay?
Pull each headlight fuse? (1 for each side).

Try each of these 1 at a time, replace whichever 1 you pulled and try the next, see what happens.

J