This seems to be a relatively common issue on the Amarok's. The MAP (Manifold Air Pressure) sensor failing, causing the car to go into limp mode with the glow plug flashing on the cluster and giving the error P0236.
Given that you have not specified whether your Amarok is a 4 cylinder diesel or a V6 diesel I cannot help with a replacement part number or exact location of the part, but it should look very similar to what I have linked to below and will have a part number on it which you can use to Google and find a replacement.
Darkside Developments - 2.5 BAR MAP Manifold Pressure Sensor - Small Probe - 038 906 051 B
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