I finally figured out that pressing the little button next to the LED in the panel near the battery causes the passenger-side fuse of the OVP to blow instantly. What the heck?
A week ago, I was able to get into diag mode but the LED just went to steady on, rather than flashing on the initial press. Now it won't go on, and I proved that the button is popping the fuse.
Without diag mode, I see battery voltage on pin 3 of the round test connector.
I am trying to fine tune the mixture to finish the recovery of this 1988 190E 2.3

Of course suggestions are welcome!