I just finished this and thought I'd share my experience. Finding the wiring diagram for the ecu was probably the hardest part of this install, on the whole it was pretty simple though.
First off remove the shift boot and cover, then remove the bezel around the stereo by pulling out the two screws at the bottom, then carfully popping the top out. Unplug the environmental controls and set them off to the side.
Remove the stereo and unplug it.
Wire the red Defi wire to the smaller diameter blue wire on the radio harness.
The black Defi to the black on the radio harness.
The white Defi to the Purple on the radio harness.
The orange Defi wire to the green w/ orange stripe on the A/C harness.
Turn the ignition on to make sure everything works. Now pull back the carpet on the passengers side to access the ECU. Remove the metal cover and the nuts holding the ECU in place, then unplug it from the harness.
I found this image on Nasioc. Thanks to winterbeater.
Attach the blue Defi wire to the Tach wire and the orange Defi wire to the Speedo.
Reinstall everything in the passenger compartment and run the defi wires through the center pedestal. Now reinstall the radio and pass the wires into the drivers side.
Now hook everything up, mount the projector where you want it, and attach the film to the windshield (ginormous pita). That's it.
The velcro is temporary, the kit comes with double sided tape.
I like it so far. I'm going to hook up the defi control unit so I have boost, oil temp and press, and EGT. I'm not sure I like the present location, but on the whole it beats having a bunch of guages imo.
This thread walked me through most of the wiring.