Can I use my car's R L rear speaker feeds (+ -) to source audio input to a bluetooth transmiter?

[kaynard]

The tramsmiter needs the equivalent of R, L line input. The car radio does not have an audio port for head phones and there is no room under the dash. Rear speakers are easy access. ,

 

[wip99gt]

I'm guessing the transmitter uses rca plugs. If so I'd use the front speakers because the rear is usually filler and you'll miss a lot. An easy way would be to purchase this:
http://www.ebay.com/itm/Kicker-KISL-2-Ft-2-Channel-K-Series-Speaker-Cable-to-RCA-Adapter-/291054465782
Then if you need a headphone jack then get a RCA to headphone adapter. Going straight speaker to headphone requires a bit more work to do as I don't think there are really any companies that make one. Some big audio enthusiasts do that for really high end headphones but I have no experience in it.