making an aux input on a saturn radio

I am trying to make an auxiliary input on a saturn oem cassette radio. Read on the internet to put a relay between j71 and j72. Common for the feed to the radio. Normally closed for the factory signal. Normally open for the auxiliary signal. Wired the coil through a switch. I am having problems with the auxiliary signal. After cutting a 3.5 mm cable, there were 3 wires. Red tip. White ring. Black shield sleeve. After hooking the red and white up to j71 and j72 it sounded horrible. Did not know what to do with the black? Then tryed it using white and black leaving red off. It sounds pretty good but that does not make sense. Can someone help me with this connection.

Ground the 3rd wire to chassis of radio or any ground pad that the chassis is soldered to the circuit board.