The ECU+ is offered in two different configurations - wire-it-yourself, and plug-n-play (PnP). If you're not good at electrical "stuff," or are just afraid to modify your stock ECU harness (for warranty or other reasons), pick the PnP unit (shown to the right). With the PnP unit, you simply unplug the four stock ECU connectors and plug them into the ECU+. Then take the ECU+'s outputs, and plug them into the stock ECU. Basic installation is literally a 10 minute job.
If you're decent at cutting and soldering wires and want to save some money (or have a 1G DSM or Galant VR-4 - PnP ECU+'s aren't available for those models), you should get the wire-it-yourself model. With the wire-it-yourself model, you solder the ECU+ wiring into the stock ECU harness. This takes an hour or two, but isn't inherently difficult. Simply follow these steps:
Remove the stock ECU. The stock ECU is behind the radio in the center console (DSMs), or behind the glove box (Galant VR4's and the EVO8). Two or three bolts hold it in place.
Using the ECU wiring diagram in the ECU+'s manual, locate the 3 (or 4, if you're using the simulated rear O2 sensor) wires that you'll need to cut. One at a time, cut the stock ECU wires and solder them to the ECU+'s wiring harness. Wrap 'em with electrical tape.
Next locate the other ECU wires that the ECU+ "taps." For each wire, strip back a little insulation and solder in the associated wire from the ECU+'s wiring harness. Again, wrap the connection with electrical tape.
Plug in the supplied serial cable to the ECU+ head unit, plug the head unit into the wiring harness, put the stock ECU back in, and bundle 'er up. You're done.
To see some examples of how to make these solder connections, check out the web site of MM XPress.