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The defrost heater should be directly under the evaporator/freezer coils.
It may extend up the evaporator sides some.

Sorry still cannot find any tech data.
Tried here, the Sears parts site and even Googled it with no luck.
If it is not a North American unit I probably will not be able to get info.

Anyways below is my usual blurb about defrost problem which may be useful to you.

First remove the evaporator cover in the freezer so you can see the coils.
If they are heavily iced/frosted over then it is a defrost problem.
Do not let them de-ice.

If yes.
Manually force a defrost cycle by turning the defrost timer cam till the fans and compressor turn off.
There is usually a hole in the cover to let you do this without taking anything apart.
It is usually either in the control console in the fresh food section, behind the front kick plate or at the back in the compressor area.
Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the timer needs replacing, re: it is probably stalling during it's rotation so is never getting into a defrost cycle.

If not on.
Unplug the unit.
Remove the wire for one side of the heating element from the wiring and measure it for continuity, usually around 20 ohms or so.

If the heater is OK
Remove one wire to the defrost thermostat and measure it, should be 0 ohms when frozen. It will be clipped to the evaporator coils at the top. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.

If both the above are OK then odds are the defrost timer contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.