A standard diode is just fine; it shouldn't be passing any significant current (as long as your main fuse in the yellow wire doesn't blow!)

The negative transients can be big; to be safe, get a 1A, 200V diode (1N5403? Can't remember the one we use in the diode leads offhand).

You install it in the orange wire (ignition sense), with the stripe towards the empeg. Rio-car's already have this - all units had transient protection, the mk2 was just missing reverse voltage protection on the ignition sense.

Hugo