Audio Systems: New head unit with old equalizer booster, sony radio, negative input

Question
I have a 70's car that had the original head unit, I had connected an equalizer booster (Audiovox) to it back than. The head unit finally gave up working, can't find parts for it so I decided to buy a new one. It's a Sony reciever/tape deck. The booster has only 3 wires for the input signal (1 negative, right and left channel positive)and the output wires that go to the speakers are paired with one negative wire for each pair ( 4 speakers).
I connected the two positive input wires to the positive high output wires coming from the Sony, but when I connect the two negative output wires to the negative input wire from the booster the Sony starts clipping and shutting off. The Sony has a negative wire for each positive wire that go to the speakers. I am only using the wires for two speakers from the Sony to connect it to the booster.
I suspect that they have different types of grounding circuits. Is there a way to connect them safely? I would like to keep the booster since it's  powerful 4 x 30 Watts. Real watts not RMS.
Thanks for your time, Vaggelis.  

Answer
RMS is real watts
you can only use one of the grounds from the sony radio
the way the system was back 20 years ago was diferent
the other option is to use only one wire from each channel of the sony radio and the ground to chasis of the sony radio
good luck