My son just bought an '08 Scion tC with nav, which has the 7.1 DVD. No override, as we expected. We tried my TCHs (old) 5.1 DVD, and and the new "Hybrid DVD" ..it would not recognive either. We pulled the radio fuse, and it should have reloaded the firmware, but no go. To confirm, we then pulled the fuse again, and it loaded the firmware for his 7.1DVD. It appears that there is a difference in the base systems, or they have gotten cleaver, and we can no longer make the "Hybrid DVDs" ....for 7.1 anyway.

What we did, however, is put in a very simple switch, that effectively cuts the speed senser wire to the NAV when switched, that allows entering destinations while moving, then he switches back, and all is good. If anyone wants more info, just let me know, and I can give the details on what we did on his Scion. I would imagine this would also work on the TCH.