If I remember correctly, there was a change in design. Originally they went on front baffle where the intake to carb heat/cabin heat duct is. Then there was a change from vans to put them over the cabin heat duct coming out of the heat muff to keep bugs, debris, etc. out of the cabin. I believe the idea is that you want nothing to restrict the cold air supply into the heat muff like ice or other objects that would stick to the screen thus starving the heated air to carb. If you have fuel injection, this isn't a concern.