The heater resistor pack controls the speed of the heater blower by limiting the voltage.
The resistor pack found in the 451 is similar to the one found in the Roadster.
These particular types have a thermal cut out fuse instead of a solder bridge.
The resistors are surface mounted, so far, they seem very reliable compared to the old type.

The resistor pack is found on the left hand side, under the dash.
This is the same for LHD and RHD 451 fortwos.

Just behind the cabin filter hatch, you will see a white base with a wiring connection.
This is the surface mounted position of the 451 heater resistor pack. 

Squeeze the 2 tabs in on each side of the electrical connector, then pull to release.

The resistor pack is now accessible for removal. Don't remove the resistor pack without disconnecting it to reduce the chance of getting an electric shock.

Twist the white base clockwise to unclip it from the air duct housing. 1/4 of a turn is enough.
Now you can pull the resistor pack out of the hole.

The resistor pack is angled towards the front of the car. The shape helps fitting and
places the thermal fuse right inline with the hot air coming from the heater matrix.
If the air is too hot, the thermal fuse cuts the power to the fan.

There are no user serviceable parts on this device.