Enzio, many thanks for your help!
Your right about the brown wire. I measure 12V if the light switch is activated. But I'm not sure what should be attached to this single pin connector...
But I will leave it as it is. Parking light is working anyway. I needed to rewire the parking light from the side markers to the headlights because I have to use european headlights and they have a integrated parking lights; the side markers are now used as additional turn indicator light.
About the purple wire: 74 Mustang has a breaker distributor, that makes me wonder. I will check if the wire is activated by ignition key or something like that. I still think it has todo with engine/emission or something like that. So your hint with the distributor module isn't perhaps that bad...
I checked the plan I found for 75 and there is also a "pink-lt blue" cable for the ignition module:
I also checked the distributor from the V6 engine that came with the car, but there is just one wire:
But I'm not sure if this is the original distributor, because its a Bosch unit...
I will check in the next days the old carb from the v6 if there is any solenoid except the coke that could connect to the pink/purple misterious wire...
The wired/wierd journey will continue.