Dan,
I believe LasVegas has one in his truck (see above).
Also, this is from the description of the changer:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">* Supports CD playback in 2005 Hummer with Navigation
*All new 2005 Hummers with Navigation, limit your ability to listed to music and operate NAV at the same time, Our SCD-G7i 6-disc CD Changer allows Hummer Owners to listen to CD and Navigate at the same time. GM does not have a factory solution.
</div></BLOCKQUOTE>