I spied this unconnected "brown" two-pin connector again, located on the fat engine compartment wiring harness that goes along the firewall, located between the brake master cylinder and the other connectors for EGR and butterflies. I don't think it's been adequately explained in several forum posts that asked about it, so after some investigating, the connector is the "VAPS data link connector" (C1041), for testing the variable-assist power steering. The brown wire is "data link input" and the white/light brown (or blue?) is the "data link output", and both wires connect directly to the VAPS computer module. One might assume there is some special tester needed, but it turns out that you can do a VAPS test by grounding one pin, and then for 90 seconds the steering effort will go between high assist and low assist. The output pin is normally 11-14 volts. At the end of this 90 second diagnostic routine, it does a little voltage dance between 12 volts down to 0 volts, perhaps to show that the voltage controller or computer is working, or simply signal that the test is over.