Does anyone know what the 4 black wires in the front wiring loom are for?

There is 1 thick wire and 3 thin wires.

I am assuming the following:

1 x thick = headlamps
1 x thin = left indicator
1 x thin = right indicator
1 x thin = wiper motor

The previous owner of my Mini was Dr Frankenstein.

I should have probably bought a new loom but I'm beyond that and only need to work out where these 4 wires connect to.

Thanks

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1966 Morris Mini Minor Saloon (Dot) - 1275 Auto - Mongrel
1968 Morris Mini Van (Ivan) - 998 Auto - Mongrel
1968 Morris Mini.Matic (George) - Original
1968 Morris Mini.Matic (Charlie) - Original

Black were all earth wires on factory cars.

That's right. Based on the wiring diagram I am assuming that they go to the above places - as earth.

Other bits like the horns seem to earth directly to the body.

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1966 Morris Mini Minor Saloon (Dot) - 1275 Auto - Mongrel
1968 Morris Mini Van (Ivan) - 998 Auto - Mongrel
1968 Morris Mini.Matic (George) - Original
1968 Morris Mini.Matic (Charlie) - Original

There isn't a lot that earths direct to body other than engine.
Even the horn and lights generally have earth leads.