Probably black & silver, a bit smaller than fag packet, 'bout £35? Well thats just a transitorised switch which sets off the spark but it is controlled by the larger box of tricks below it which is the main ECU. Whist the ignition module makes the spark by switching the current to the coil on and off (very quickly) the ECU actually controls 'when' it makes the spark and is controlled by RPM, vacuum and temperature......oh god this is getting heavy.
A small diameter transparent vacuum pipe is fed to the ECU, if this comes disconnected or splits somewhere it can mess the ignition timing up badly especially at higher revs. Also there is a temperature sender on the gearbox end of the cylinder head (there are four senders grouped together I dont know which one it is) which feeds to the ECU, if this is faulty it can also affect the ECU ignition timing.
I'm still curious as to how the original ignition module failed, did it just 'stop' working?. If the new ignition module is def OK and it seems you've replaced just about everything else then the problem could be in the main ECU or the bits connected to it. You've spent loads of cash so far, some of it wasted & got nowhere, I could not suggest that you change the ECU, but you might check the vacuum pipe (its a long one) and you might try swapping the temperature sender, if you can figure which one it is! (Help! someone must know which one it is). You might just have to bite the bullet and take it to a main dealer or VW specialist to sort. What I can't figure is why you should be left with a problem if you replaced a part which was obviously faulty. Maybe the failure in the IM shorted something out in the main ECU........
