Re-Fitting into my caravan a Truma S3002?

Having purchased a second hand Truma S3220, the unit was shipped to me in three boxes! The core heater, the front bezel/panel and a rear panel. I cannot fathom out how to put it all back together again and fit it into my caravan which prior to my purchasing it, must have had a Truma S3002 fitted. I will presume it was taken out and discarded at some time as faulty. Anyone able to help or where I can find an "exploded" view of the full unit please.