Hei /6 won't shut off?

Somehow I missed this was an overseas car. Yup!! Different alternator and different wiring. It surely is related to the alternator / indicator lamp I would say

The diode you would need is very non critical. Any rectifier diode rated for 25 PIV "or more" and say, 1/2 amp or more. Depending on the harness setup, you might be able to build a little heat shrinked assembly that will plug into the indicator circuit.

And it will depend somewhat on how the alternator is wired. It would be helpful if you can provide a diagram of that.

This kind of thing, really, is poor design on the part of the manufacturers. What they depended on, originally, is that the feedback to the ignition line is "not much power" and that due to the relatively large current required by the load on the ignition "run" circuit, it would be dropped / bled off to a low enough value, that it would not be a problem. People "come along" with relays and MSD etc and now the load on the run circuit has dropped to a low value (relay coil) so now it "IS" a big problem.