I have a harness to use in the car that has a loop for the belt to go through on the back, its from some dutch company so you propably wont find the exactly same harness as I have in the USA, but you can look for some that have a loop in the back to attach harness !
I'm sure someone here can tell you about a harness with a loop hole for the belt to go through.
__________________ Sun [Owner]  , Fido [Yorkie]  , Mímir [Chinese Crested]  , Kría [Icelandic cat]  , Skuggi [Forrest cat] |