I haven't verified that these two share the exact same interface, but they do have the same connector with the same pin assignments, both using Matsu****a-manufactured head units.

By swapping a connector, an add-on single-CD player from a '90s Honda will function perfectly with the Mazda head units, except that you have to "trick" the head unit into treating it like a CD changer by reselecting disc 1. See the procedure here.

Now, without an o-scope to verify what's coming out of the head units, what follows is conjecture.

The manufacturers of the various aux in/iPod/carPC adapters for '90s Honda head units claim that their devices will not work with the head units designed for the single-CD add-on that works with my Mazda. They'll physically connect, but they won't function for whatever reason. Does anyone know what that reason is? Do the Honda single-CD players use a stripped-down version of M-Bus, or is it another protocol entirely? It must be some sort of standard protocol if my Mazda is able to talk to it, albeit in a stilted manner that requires cycling the disc selection button.

For that matter, and the real crux of my question, has anyone successfully interfaced with the OEM head unit in one of the cars mentioned in the thread title? They'll have a 16-pin (8x2) rectangular connector with this pinout.