You can use almost any silicon diode, cheap choices are 1N4001 through 1N4005, commonly available for less than ten cents in any kind of quantity.
The opo-isolation is only on one of the modules, and also the input LED for the opto has a reverse voltage specification, I didn't check what it was.
As far as where to install them, I thought I said that a couple of times.
you just need a series diode with the cathode toward the Fastrack switch indicator lamp lead to block the negative voltage.
The cathode is the banded end of the diode.
