First off, suggest you not involve the coil when doing this experiment. It’s tough enough to solve elecgrical problems without introducing add’l components into the circuit. The negative terminal of the coil on a points-type ignition system sometimes is connected to ground via the point contacts. In that case the fan wouldn’t run at all. When the point contacts are open though it will measure close to the battery voltage, but there’s a current path through the coil involved. Not a good idea to power the blower fan through the primary of the ignition coil. That’s too much current for too much on-time, might burn out the ignition coil. Plus you’ll likely get a big spark whenever you disconnect it.
I don’t know how the 52 Merc heater blower is wired, but on all of my vehicles tI’ve ever owned, there’s a fused battery + circuit which goes to the heater fan resistor block, and the switch you use to set the fan speed selects from among various points in that resistor block (a network of resistors) , and directs that particular selected contact point to the + of the blower. For high speed, it is just the battery + that is selected. For slower fan speeds the path goes through some of the resistors. Usually the path that burns out is the one for the slowest fan speed. That’s b/c the power is divided between the fan and the resistors, and at slow fan speeds the most power goes to the resistors and the least power goes to the fan. The blower negative is connected to chassis ground.
If you measure the voltage using a volt meter between the fan + and chassis ground, it should be battery voltage in the high speed setting, and a lower voltage in the slower speed settings. What voltages are you reading at each fan speed setting when you measure between those two points?