add stereo headphon jack to rear speakers

I have a 96 Nissan Bluebird (Altima in US I think). I wish to wire a stereo jack into my rear speaker wiring into which I may plug stereo earphones or a bluetooth transmitter. The jack has three terminals, one left one right and a common negative. The car's negative left and right speaker wires are different colours. Can I pick either of these to be the common negative for the jack or do I earth the common negative of the jack to the chassis, thanks Dave

Best thing to do is use a chasis ground. Since they share a common splicing into one speaker is ok but chasis is better.

STOP. There is 6 volts of DC on every speaker wire in your car. This can be done, but it will require 2 non-polarized capacitors. You can ground the common wire to the chassis. You MUST install the capacitors between the positive wire and the headphone jack. The capacitor will not allow the DC voltage to get to your headphones. I can draw you a diagram if needed.

Thanks JR and IAAI, a diagram would be great, thank you very much, Dave

My advice is to not try and tap into the speaker wires but to use a 12V line level amp and take the signal off the RCA outputs of your head unit (or add an LOC if your head dies not have them.)  Use a 12V headphone amp (something like the Boostaroo R234 can be had for $50) and you'll have a more reliable and safer solution.Support the12volt.com