Inspiron 620, not the same as the OptiPlex 390

Wire Tracing is possible and relatively easy when you have the correct parts.

  You will have to reverse engineer the part to find out exactly.  Removing the LED's allows you to visually see the polarity.

The front panel has 6 wires AND A Jumper?  Part Number KCRV8

You should be able to trace the wires from the assembly to get pinout.

Also do not confuse the INTEL Standard at the bottom this diagram is NOT the 620 pinout.  This is more than enough data to get you going.

If you want all the ports I would suggest buying and tracing a Dell 620S front panel with USB SD Reader DELL K6M7C.

https://www.amazon.com/Dell-OEM-Original-Part-Computer/dp/B00EKLRROC

 

 

 

INTEL PINOUT Below  This IS NOT Dell Standard.  May be similar to Dimension 8500/8700 however I don't have one so I cant say one way or the other.

You will have to trace for the USB as well as the Audio cables.

USB is Green Audio is Yellow Front panel is black.