u can use the existing earth thats no problem ok let me see if i can explain my self better. the way you have described it
anyway me being me I took the nobs advice at jb and brought the iso plug that connects to the stock holden plug then I just need to soldier wires onto the headunit cable. I'm sure il have a few hickups along the way.
this is the way to be.. if u dont wanna go this way, the below explaination is another option using a patch harness.
vx audio loom connects to the areopro connector lets call this end, end1.
the only other way to have a patch harness so u purchase another matching plug&socket. one end of the harness u solder/attach to end2 of ur areopro connector.
the other end of the harness u connect/solder/attach to the headunit wiring.
i assume that the headunit has barewires and no plug at the end of the wires? (forget the plug on the back of the hedunit itself) if so use the patch harness. it may be a bit bulky but will be ur best bet so u dont have to cut anything