I followed this guy and did the Rallye 3, the Rallye 4 may not work since the rear seats are so close the angle of the straps would be extreme. Might be okay for autocross though. The 3 uses a single attachment point which can be attached to the seat belt reel near the C-pillar. It's at an angle to the seat but that's still an approved way to mount it. You can also mount it to the child seat anchor to make it straight but the belt points are stronger.
http://www.ft86club.com/forums/showthread.php?t=15051
There are a few folks making harness bars now but most are more like rollbars...