Two things
1. The app should still work - it works in my truck, but maybe Teyes changed the firmware and broke it. I'm headed out of town, but can look into this next week, but will need some help from someone with the problem. I'll need to send you a custom version of the app, have you run it and report back, fix it, retest, etc. Anyone up to help out on that? Alternatively, if one of you is a software engineer, the app is open source and you can just debug yourself and submit a pull request!
2. The easiest way to check the rear speakers is to look at the wiring harness pics I posted in that other thread. If the harness wires aren't in the correct position, then you're not getting rear-audio out of the teyes. I suspect that the stock amp might be leaking front speaker signal into the rears and that's why people get sound from the rear but no ability to fade. I have an aftermarket amp that keeps front and rear separate so I was getting zero sound from the rear (until I fixed the harness)
Hope that makes sense!