I'm a bit dubious about how that wire would turn on a check engine light. The yellow wire is serial data from the ECU to the BCM it also connects to the OBD2 port. It's a digital data line used to transport messages back and forth between the BCM and ECU. From this data the BCM runs
oil pressure and water temp guages, trip computer and a bunch of other stuff. The OBD2 port uses it for flashing the ECU or pulling trouble codes out of it. How will it light an engine check light if it's a data line? It would need to put out a hard 12 Volts on a trouble code.
Have you tested this, how reliably does it work? It should be reasonably easy to generate a condition that would bring up a check engine trouble code. Disconnecting one of the O2 sensors or the TPS or something like that would be enough to prove it.
I've wired the yellow lead to an OBDII port that'll be tucked under the dash. It'll make reflashing the ECU easier because I won't have to unplug it. There are also a variety of OBDII scanners and guages that will tell you the trouble codes coming out of the ECU and a heap of other useful information.
http://www.aeroforcetech.com/
http://www.scangage.com/
Cheers