No problem. I didn't mean to jump you either. It's just that I've seen dealerships do the same thing you've done and charge people $400 only to have the problem come back with in a year. The only thing I can think of why it works for a little while is from routing around in there with the wiring may move the decoder (even though it isn't suppose to) enough to get it to work again. But, after a while it goes back to the way it was.
As far as unhooking the system and putting the resistor in the end of the connector, not sure if it'd work or not. I'm not totally sure what the white wire that's with the other 2 have to do in the system. In the old PK1 systems like in a '98 Sunfire use that wire as a bulb check in the security system. If that doesn't check out, it'll through a code. Also, the black wire is a ground reference wire and I'm sure there would be a little bit of change in value if you unplug that wire. Reason is that grounds in these GM vehicles are horrid and usually are very poor.
If you start having the same problem, just try it and see.