I understand the diode to be that item at the top. It did not come with the re-man unit I bought, that one was on the one I traded in.
Not sure where you'd pick one up.

Diodes are sort of "one way" Electrical valves
 

Alternators produce AC power (Three Phases) So, It's my understanding that
3 separate Diodes are needed for "Rectification" to DC power.

The item at the top, in the photo, is a noise filter. It's usually an add-on.

The diodes are internal.

^ Thanks.

Just to finish up what I consider the conclusion of this thread on my alternator. It's been almost two months and 1500 miles and my alternator and battery are fine. So what have we learned. Sometimes a device will test bad on one machine and good on another. Not the first time for me or the last. Try and look at the whole picture before spending MONEY. I'm happy, and my car is happy. Thanks for all the input. Thanks

I had the diode test bad on my 2000 diesel ram. Batteries are new and good. I also drove it figuring that as long as it is charging, it's not broken. On Tuesday of this week, I'm in a small TN town & my dash volt meter drops to 0 and "check gauges light" comes on. Bottom line, instead of changing a few $ rectifier at my convenience, I changed a $120 alternator with borrowed tools in a parking lot. Fortunately, I had some workout clothes with me & didn't have to change it while wearing office clothes. Local shops were busy & didn't have time to do the work. Fortunately, the store loaned me the tools & I didn't have to buy them. Change your diodes.

In the pic it looks like there is 9 Diodes in the Alternator and it looks like 6 have an effect on Rectification.

The six at right are a typical rectifier circuit, but if one of the three to the left fails, you'll start getting AC into the stator windings ("field coil") which will adversely affect charging.