I know that its easy to find the entire E30 wiring diagram on t'internet but never looked for e36. However that may not be of much use as they don't always show indivudual connections. I would start by taking a multimeter and checking what are the physical connections of the indicator switch. It should be straightforward (ish) after that.
I used a stripped down original BMW loom. It was a lot of work to identify and strip out all the unused wires but the plus side is that I didn't have to worry about stuff like indicators hazards etc. It was all plug n play...
Cheers, Robin
PS - Found it. Have a look here
http://www.e36club.ru/faq/E36_Sedan_...after%2093.pdf