We ended up buying a double-duty harness for Akira. We bought it from the vet, but since we bought it I've had a look and found the exact same one on eBay for a LOT cheaper! (Link)
Basically the back has both a D ring that you can use for a lead, but the nylon on the back is doubled and the seatbelt goes through the hole in between the two straps. Akira was fairly placid in the car before we got that harness, but she tended to move around a bit because the lead that clipped into the buckle gave her wriggle room. Now she just sits there and can even lay down and go to sleep.