Well, if apt can't get to those headers, neither will the install script from TrendTech. One more thing I fiddled with is the /etc/apt/sources.list.d/raspi.list file in which I think I uncommented the three repos. Could you try uncommenting those, re-run a sudo apt-get update and then try again with the rasberrypi-kernel-headers (or go straight for the TrendTech install process which does the same thing at some point)?