I think a lot of people were trying to figure this one out a while ago. It turns out that all the software for bluetooth phone capabilities are already on the 4th-gen NAV systems, but the hardware that receives and sends bluetooth information isn't there.
Check out this thread:
http://www.toyota-4runner.org/showthread.php?s=&threadid=10663&perpage=15&pagenu mber=2
Hope it helps!
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2003 4Runner SR5 V6 4x2 - Pacific Blue Metallic/Taupe Leather