Sounds like you're unlucky, as I have almost the exact opposite experience... that said, my GM rentals tend to be PXAR and up, and even the highest mileage GM car I've had in the last 12 monthe (a '13 ATS over 20k miles) had working OnStar (but no SXM).
How is your experience with your eBay Ford Nav SD Card working? Does it need to match the model year you're driving? I've thought about going this route but hadn't had a Ford rental recently... then last week I got a '15 Navigator on an LCAR res at FLL... only 242 miles on the ODO, but no SD Card. If I could buy a '13 or '14 card and expect it to work for a couple of years, I really might consider it... if I'd need a specific card for every model year, I probably wouldn't go through the hassle.
On an unrelated note, I recently had a '14 XTS at LAX that only had On-Star (no stand-alone Nav). It had previously been reported that the XTS was stand-alone... is there a trim level differentiation, or is it On-Star only? I'm going to ad "???" to its designation for now...