Hope they've improved since they started this program a few years back; I *never* got credit for parking, even once, even after calling and faxing receipts. Of course, their regular car rental operation didn't credit about half the time, either. I've given up on Thrifty, pretty much.
Thought I might try again now that they've got "Gold Points" to give (Radisson's program). Apparently you can get a significant number of Gold Points instead of miles for rentals. Since the miles are now of insignificant number, this might be a reason to try it again.
I'd be interested to hear about parking experiences from any Thrifty SFO parking customers these days.....