Fixed it!
I did your test light idea. It worked.
(not sure why the alternator was not outputting the other day...? must have been my test light?)
Then I ran a wire from the D terminal to the bulb on the speedo. The light went out!
I was also able to unhook the battery while the engine was running and the engine kept running and did not die this time. So the problem all along was the wire to the speedometer is bad/broken/shorting out.
Going to run a new wire and everything should be fine. Thanks for the advice!