I am putting a Bluebird motor into a 2001 G20, and was wondering how to modify the existing harness (similar to a B15) to use the plug and ECU from a reprogrammed '94-'96 ECU...anyone done it before?

you would need to splice in the ecu to the harness, they have different connectors and sensors.

www.b15sentra.net might have more information if you do a few good searches.

here's what Travis (unlucky) told me has to be done for the JWT ECU to work in the B15....

entire ECU harness has to be repinned
MAF plug changed
o2 sensor wiring changed
IACV changed and wiring
FICD changed and wiring

might want to check with him for specifics.