At the third try, with me actually putting in an order, I finally got the correct wiring harness. To give the vendor their due, they refunded the cost of that order; my final total paid was the same as if they'd sent me the right wires in the first place.
So yesterday afternoon, I sat in a lawn chair and exercised my crimping skills to make a wiring harness. It ended up a yard long, almost unbelievable to think that it would disappear into my dashboard. But, disappear it did, after I'd removed various pieces to expose the necessary connections. As a side note, if I ever find the chap who decided the connections to the rear speakers of my car should be made above the driver's left knee, I would advise him to call for an ambulance immediately; the sheer volume of shouting I intend to direct at him will likely burst his eardrums!
This oddly-located connection meant that I was upside down, laying across the door sill of my car (thankfully I'd found our one-step hop-up to support the parts of me outside the car) with my head in the footwell, one hand reaching deeply into the radio recess with the plug in hand, and the other hand attempting to sneak into the space just above the kick panel to get a hold of the plug to pull it through. Once I'd got it through, I decided to bite the bullet and commit; I lopped off the old plug to pull that harness out. Then it was simply a case of making sure all the remaining connections were solid, and reassembling the dash, and I could reconnect the battery and try it out.
My car now speaks iPod. As a bonus, the new stereo sticks out less than the old one.
I shan't be installing a new stereo any time soon, though. If it becomes necessary, I'll have a professional do it; I found the experience rather painful in multiple ways, not unlike being a combination contortionist and OB/GYN to a box of dull knives.