Aux sound problems

Iahri, im trying to make an auxilary input for my pc which has onboard realtek, but all i get is the left channel working.
If the aux balance is centered or to the right, the pc sound cuts off to the left channel as well.
Can anyone please assist me with this problem?
I have: 4 pin plug which has 3 wires, red(right), black(gnd) and white(left).
Also i have 2 inputs (for audio left right) which i have tried to connect with the 4 pin wires.
The 2 inputs lead to an rca plug (stereo).
This works great when i plug it into line in, but there is a space open behind my pc which i wish to fill.
I have tried to make 2 black(gnd) wires to complete the 4 pin plugs 2 middle spaces but with same results.
Anyone have an idea?
Many thanks
dJ

To better understand what plugs i want to use to create the aux in, heres a pic.