I recently installed turn signals on my P15.
I had a wiring diagram from the signal-kit. But al the colours were faded... & I didn't had the time or motivation to seek all the proper wires in the car.
So I rewired the car with turn signal-wires. (just 4 wires... that's all)
This was much easier and faster!!
then off course, there is a lot of trial & error... but with a multi-meter (or how do you call it),you can find out which wire sends a left or right signal out.
by the way, the flasher... is it a model with 2 or 3 connectors?
However, I suffered from other "problems" after that...
Just click the link: Flasher troubles -> click here.
Hope this will help you!
Regards,
Kurt