Because of where the blower motor is mounted. A single coil ballast would work for the low side well if I just put it inside the area above the wheel well behind the outer quarter panel.
The problem is I have no idea whether the resistance is correct. What you need is one of these hoarders on here LOL to measure one they have that is for the defroser
Used to be parts stores sold "volt drop" various names intended for 6/12 conversions. If memory serves, one popular "round" device was designed to drop 12 to 6 WHEN USED with a 4A load. The motor load and the resistance both determine how fast it will go, along of course with supply voltage
