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Check R-D Racing. I have their bar with a 5 point harness. The bar attaches to a couple of different points. One is where the stock belt loop point mounts and the other is under the cover for the targa top storage slot behind the seats on the wheel well.
The one from R-D installed pretty easily and you still retain pretty much all of the seat fore / aft travel.
+1 for R-D Racing Their harness bar is well made and very reasonably priced. Dave is a great guy to do business with and he's a CF Supporting Vendor too. In fact, he sponsors the C4 Tech section.