[jcassity]

so... i see that now,, i dont have a phono input on my Z and it took me a minute to see that in the diagram you posted, its a slider switch.

here this guy is also agreeing that within the modern smartphone headphone jack there is a left and right speaker signal then deal with the ground & shield as required.
he is agreeing that you join up the modern cable left and right with inline 1kohm resistors on each wire so your smart device interior electronics see's a load to simulate ear buds.

in this vid,, there is an OEM toggle switch so to speak by way of how the radio was designed.
https://www.youtube.com/watch?v=kL94do8x1-g

in my case i hope to find the jack that acts as contacts so when the cable is pulled out, i am in "radio mode", then when the cable is inserted, i am in ipod mode.

there are lots of discussions on isolation transformers as well incase you want an insurance policy against the possibility of a hot chassis.,, this gets into the discussion of earthing vs grounding vs floating chassis etc...

in m case i will mount my jack to a non-conductive surface ,, this way i simply intercept the wire that is tied to my vol control.
what i know is we dont intercept the wiper..........
we intercept what is called the "high side" of the vol control. so i think that means that looking at the front side of the vol control, you would always use the "LEFT" terminal lug.