The OBDII harness only uses four conductors. Just tap those under the dash, and make a directly connected serial cable to tap them out of sight. There' no mucking around in the ECU harness or going through the firewall, plus the OBDII port remains unpopulated, and available.

I got this idea from Papa Smurf's OBDII relocation kit, which I also have.

-Skydive
2001 S4 Avant - Now Returning to Stock!

450 Vertical Feet in a B5 Audi Chassis
Plan the Work. Work the Plan.