I think you are correct. Sparco do a harness bar for behind the seats, but you'd need a short harness for it as it runs straight from the bar directly behind the seats through the seats. Take a look on my350z.com, loads of guys run them on there
Z1Auto suggest that there are long (4 seater) and short (2 seater) versions, so more weight to needing short harness ( http://z1auto.com/prodmore.asp?model=35 ... rodid=1752 )