The very first split out of the wiring harness from the ECU is the C104 connector, which is supposed to run to the instrument cluster. Because the 325e and 325i connectors are different, you'll have to do some splicing or bridging. I wanted to bridge the wires since splicing can be messy and can cause some problems later on for someone else.
If you disconnect the harness from the ECU, you will see 55 pins with the sequence running from right to left, as follows:
Pins 1, 20, and 38 are adjacent to the wires and pins 19, 37, and 55 are on the free side of the connector face.
The two wires that must be bridged from the instrument cluster harness are #6 and #32: Pin #6 is the engine speed output and gives the instrument cluster the signal for the tachometer, while pin #32 is the fuel consumption output and gives the cluster the signal for the econogauge. If you have an OBC with the CODE feature active (and you still want to have it functional), then you'll also have to bridge the wire from pin #38, which is the input signal that enables the driveaway protection. So you'll have to bridge:
- black wire from instrument cluster harness to black wire on engine harness (runs to pin #6)
- yellow/white wire from instrument cluster harness to white/black wire on engiine harness (runs to pin #32).
Once you do that, voila! Everything should work nicely. The OBC, which receives its signal from the instrument cluster, will also function normally afterwards.
Thanks again to Mitch for all the help!