190E: Why does diag mode blow an OVP fuse?

[lombrozo]

Making a huge amount of progress getting this 1988 190E 2.3 running good. Now I just have a lumpy idle and off-idle to figure out.
When I press the micro-switch by the LED, by the battery, to put the car into diagnostic mode, it instantly blows the passenger-side fuse in the OVP.

I don't see anything out of place wiring-wise, any suggestions on what transformations take place when the micro-switch is pressed?

In the meantime I am trying to figure out the rough idle old-school. Yes, reading the sparkplugs. The issue is that cylinders 1 and 2 are pretty rich, sooting up the plugs, and 3 and 4 look fine.

Tonight I will try to find any sort of air leak that is diluting a rich mixture for the rear cylinders.

The fuel injectors are brand new, I have not tried swapping them. Suspecting maybe an intake manifold leak? Are there any vac lines connected to the bottom of the intake manifold that would only affect the rear cylinders if leaking?

Any ideas are welcome, this has gone on for far too long! So close, can almost hand the car over to the young lady it is built for.

Thanks for ideas,
David