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The basic difference, as someone mentioned earlier, is the location of the stoplight switch, necessitating different lengths of wiring on the leads that go each switch. The 28/29 stoplight switch mounts on the transmission tower whereas the 30/31 stoplight switch mounts off the back of the center frame crossmember. The leads for the switches come off the main harness at the back of the center crossmember at the lower frame rail on the driver's side of the car. The distance from that frame rail to the 28/29 switch is significantly further than that of the 30/31 switch. So, the stoplight switch leads on the s8/29 harness need to be longer. I know of no other differences in the harnesses but that difference is significant. That said, since the 28/29 harness has longer leads and you are installing the wiring on a 30/31 car ............. and assuming you have a 30/31 stoplight switch mounted on the center crossmember ........... it shouldn't matter which one you have. Either year harness will work on a 30/31. Not so for a 28/29 application.