Ok, you can get a schottky diode here:
http://www.maplin.co.uk/Module.aspx?Mod ... y&doy=11m7
Any one of those will do - they all drop around 0.3v. You also need a resistor too - probably anywhere between 10k and 50k would work. I wrote 10k on the diagram, but now I think of it nearer 20k would be better.
Then measure the voltages on the 3 wires to the MAP sensor against the engine chassis (the Vec and VX220 had different colour wires, so I won't give them here). There should be +5v(ish), 0v(ish) and something else (signal) that's almost near 5v, but isn't. If try with the engine running (plug still connected - just poke the probe into the wire) the signal one should be really obvious as it'll be nowhere near 5v any more.
Then... Cut the signal wire, and strip a little bit of insulation off of the 0v wire. Connect the Schottky Diode across the 2 signal wires, with the stripey end pointing towards the ECU, and then connect the resistor from the stripey end of the diode (ecu-side of signal wire) to the 0v wire, and insulate everything. With the ignition on and engine off, you should be able to measure around 4.6v between the ecu-side signal wire and 0v, and 4.9v between the map-side signal wire and 0v.
I pulled off the corrugated plastic pipe stuff and fitted everything under there - then it's pretty much hidden.
Hastily created circuit diagram here:
