Well, here goes... All this is trying to do is lower the voltage from the MAP sensor by 0.2v. It should make the ECU think that there is more vacuum than there actually is, but hopefully not enough to mess up the ECU's mapping by a significant amount.
The colours are from my Vec 2.2 and may be different, so I'd check first. Just turn the ignition on and check with a volt meter, you should be able to find which wire is 0v and which is 5v. With the engine off the signal wire should be about 4.8v, with the engine idling it was about 1.14v on mine. You can easily get the corrugated plastic covering off the wires, then do the mod and hide it back inside the covering.
The diode is a Schottky diode, not a normal one. Something like a 1N5817/8/9. It doesn't have to have a high amp rating. It just has to drop 0.2v. You can get them from Maplins:
http://www.maplin.co.uk/Module.aspx?Tab ... 2#&doy=8m5
The resistor is less important. Somewhere between 10k ohms and 100k ohms is good I reckon. You could check it is working properly by checking the voltage on the signal wire when the ignition is on but engine off, before you do the mod and after (on the ECU-side of the diode). It should hopefully be about 0.2v lower.