Limited number of Bluetooth connections with one given iPhone

Hi all,
As a car fleet manager with a car manufacturer, I routinely switch cars. I sometimes drive several cars on any given day, and until recently, I hadnt had any problem connecting to the Navi system on the cars I drive. Two days ago, I tried to pair my iPhone with the Navi system in a new car, was prompted to enter a PIN, did so, and never managed to connect. Ive reset all network settings on my phone, turned Bluetooth on and off, both on the phone and the Navi system, to no avail. Ive erased all mentions of previous phones being paired with this company car, still a no-go. The brand is Nissan, by the way, and its the first time Ive encountered such a problem. Pairing is usually a breeze, not so this time.
Any ideas?
MacBook Pro 2006 Mac OS X (10.6.7) 2GB RAM - iPhone 4 16Gb - Longing for the new 17" i7 MacBook Pro!!!