Unless you spilled some in the front seat-something IS wrong.
Did they perssure test it? Or did they just visually look for leaks?
How about the heater core? You may actually have a little bit in the front seat afterall. Another place to go to check out odd problems is the CARTALK section at CARS.COM
Is the smell inside or outside the truck, inside may be the heater core ... Outside, then it maybe the seals on the tanks of the radiator, I had this happen to mine ,, when the truck changed temps it would leak a little, very slightly and would smell up front.. The radiators are three piece on the fords, the tanks are plastic and the core is aluminum with a rubber gasket between them, sometimes this gasket and the expansion and contraction of the three materials causes some leakage... cant' always detect this with a pressure testor,
definetely sounds like a heater core.Dont forget the PSD has cavitation problems if the cooling system isnt maintained.,and if the additive wasnt added ,this could become a problem in the future,if it is cavitation now,their would be water in your oil.
Cavitation in the case of INTL diesels is a case of corrosion eating a hole between the water jacket and the cylinder bore in around the #8 cylinder. INTL sells a conditioner that needs to be added to the coolant to keep this from happening. Everyone that owned INTL's knew about this, but FORD forgot to mention that little fact when printing the owners manual.