Also, I wanted to mention all you have to do is remove the Arm Cup and the removed the screws around the Battery capsule area. No need to mess with the Headpiece.
There is some screws or one under the XS side sticker.
Very Carefully open it up. Use something to wedge in there to keep the two sides open.
You will first need to pick out the Hot-Glue before you will be able to get the 1/8 plug out
You will need a Hot-glue gun from a hobby shop to hold the new 1/8 in also..
Snip the wires as close as you can because there is not much left over to play with for sure.
As far as the Common I believe it makes contact on the inside.
I truly cannot remember but, I remember looking at the inside of the one I did last year and boy are they thin and cheap IMHO.
That is why they wore out so darn fast IMHO.
You can just pop the black cap of the defective one (only after you make sure you get they wiring down pat) and you will see what I am talking about.
I wish I had the pictures still I did back here in 2000 something but, most are no longer viewable.
I was shocked even to find the picture of the 1/8 for you.
Just follow those steps and you should be fine.
As stated Test before reassembly.
Thats all I can really add to what I have already posted.
Let me know how it works out.