I found a diagram, tried to send the link but it doesn't work...
It showed 4 terminals on the switch.
1 and 2 (tied together in the switch) = start
3 = from the ampmeter 4 = preheat
If your switch is different, use an ohm meter and determine which terminal is the common. The terminal that is connected to nothing when in center, start when turned right, heat when turned left. That terminal will be the battery (from ampmeter) terminal. The other two will be start and heat.
If it still wants to crank in the heat position, look over the wiring, someone may have jumpered the heat circuit to the starter circuit elsewhere.