Allright!
Tested my prototype today and it works. Shows ~650 rpm at idle, which is about correct. The scope shows ~75ms rev duration, which is okay too. Interesting thing is that i think that one coil pack actually makes one pulse per revolution, or well it at least gets one pulse per rev from the ECU. I will have to see through my software to see again if this is correct.
I will post the code as soon as i comment and document it and schematics as soon as i make them (i was working on a development system and added a prototype board to it). The system picks up the signal directly at the LT side of the coil pack, and uses a zener diode (thanks
reddy4bed, turns out i didn't need optocoupling or anything) to limit it to ~5v (down from ~12V). Pictures coming as soon as i build the pcb, and start installing it. As i said before, i'll give you the code and the schematics so you can play with it, and quite possibly improve it. An analogue display could be an option, but i can't deal with that right now (uni and all)
Any ideas how to get the cables (not more than a few square mm) from the engine bay to the dashboard? I'm sure they pulled the original cables somewhere, but i can't see that spot. It's going to be 3-4 wires, 12V from the ignition, ground and the LT signal from the ecu, possibly from a ribbon cable, so it's not too thick.
Cheers and thanks for the help, i really really appreciate it, i know it sounds cheesy, but i couldn't have done it without you guys.