From the 986 Factory Service Manual, Audi A6 (C5) Bentley and a few old spreadsheets I made, here's a simplified diagram:
That's from the 1998 Boxster and 40 valve AWN engine A6. The 32 valve V8s will have a different ECM pinout, but I have that filed somewhere too if it's needed.
That same signal should also be run to pin 9 of the OBD port.
I have not tried it yet. I'm curious if the signal output by the Audi ECM is identical to the one put out by the Boxster's DME. I expect that it will work and not make smoke come out of the cluster.
