It seems that you have the wrong kind of plug. Although you may have the right size (3.5mm), you must have the correct number of poles for each plug.

An ordinary stereo ear/headphone plug has 3 poles ... one each for the left channel, right channel and ground. Some devices need a fourth pole, ex: a control line for the device.

If your HU needs a 3-pole plug, use a 3-pole plug at that end. Consult your HU manual for the required plug.




If your MP3 player needs a 4-pole plug, use a 4-pole plug for that end. TIP: The supplied headphones should tell you how many poles it requires.



HTH