Looks like the problem was in the type of posi-tap connectors used. Rather than the specific kind linked to in the DIY install thread, my mechanic had gotten some similar-looking connectors locally. Since the DP fix wasn't getting power, I went and ordered the exact posi-taps mentioned in the DIY thread.
http://www.crutchfield.com/p_669PP64...onnectors.html
Mechanic went back and redid the wiring with these posi-taps and ditched the other ones. The green light on the DP fix is now coming on as it's supposed to. I'm picking up the car this afternoon and will be able to do the code clearing, drive cycle, etc. and will report back. But fingers crossed that now that I have the green light on, it should be working correctly. Looking forward to no longer having the stupid SES light on all the time.