How do I keep my headphone jack from disabling my bluetooth speakerphone in my car?
From the Samsung tech support, they say that:


  when the AUX cable is plugged in, Blue-tooth audio is dead.


However, from the ICS Options during a call:


  During a call, you can also use buttons near the bottom of the screen to:
  
  
  Switch to the dialer, for example to enter a code.
  Controls the audio output as follows: 
  
  
  If a Bluetooth headset is not connected: Turns the speakerphone on or off.
  If a Bluetooth headset is connected: Functions as a 3-way switch between the built-in earpiece, the speakerphone, and the headset. The icon changes to indicate the current output. Touch &amp; hold to see a 3-way menu.
  If a wired headset is plugged into the phone, it replaces the phone's built-in earpiece.
  
  


Note:
The Google Support link refers to the Android 4.0 for Galaxy Nexus (Released 2011, November), but the one you have is newer (Released 2012, May). Since the OS is the same, there's no reason as to not have those options available.



You can try and check if the mentioned option at point 2.2. works, but besides from that, I believe that you're out of luck on this subject.