Harness with the stock seats is a bad idea. This is because the stock 4th gen seats have no place for the top two belts to rest on for support. The belts would just slide down and only be supported by your shoulders (bad idea). A harness bar would fix the problem somewhat, and is a good idea, but it is best to have aftermarket seats with harness guides built into them. Also remember to keep a 45° angle or less on the belts from the shoulder support to the mounting position to ensure safety if an accident was to occur (Spinal compression = bad).
I just ordered a 4-point Sparco Clubman harness for my drivers seat yesterday, and will be receiving it tomorrow to hopefully be installed by the end of next week. I have Tenzo Rally seats with harness guides BTW. I will be mounting them with snap-in's in case I need to take the harness out for some reason, as well as keeping the stock seatbelt in for comfort. My harness is purely for use when I drive canyons and such, so around town I will have my stock belt to wear.
Hope I was of some assistance.