well it's most likely going to just be a straight across cable, and it should use standard headphone jacks as I recall seeing, though I'll ask my friend tonite, since he loves his 32x like it's his baby. If it is a straight across with headphone jacks on both ends then RS should have one for sale, if not they could probably tell you how to make one.
I think youre confusing the Sega CD mixing cable with the 32x patch cable. The 32x cable goes from the av out of the genesis to the av in of the 32x. The mixing cable for the sega cd is only needed for the model 1 genesis to produce stereo sound out of the headphone jack.