der Fond
rear seat
The rear seating area in a passenger car.
Die Kinder saßen bequem im Fond und schauten einen Film.
The children sat comfortably in the rear seat and watched a film.

rear seat / stock / background
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rear seat
The rear seating area in a passenger car.
Die Kinder saßen bequem im Fond und schauten einen Film.
The children sat comfortably in the rear seat and watched a film.

The word 'Fond' comes from French. Therefore, the final letter 'd' is not pronounced, and the vowel 'o' is pronounced nasally.
Bitte nehmen Sie im Fond Platz.
Please take a seat in the rear.
The three meanings are clearly separated by context. In connection with cars, it refers to the seating area; in cooking, to the broth; and in art, to the background.
Die Papiere liegen hinten im Fond des Wagens.
The papers are in the back, in the rear seat area of the car.
When referring to the seating area in a car, you use 'im Fond' (dative) for location and 'in den Fond' (accusative) for movement toward it.
Die Jacke liegt noch im Fond.
The jacket is still in the rear seat area.
In cooking, 'Fond' is often used in compounds such as 'Gemüsefond' (vegetable stock), 'Fischfond' (fish stock), or 'Kalbsfond' (veal stock). Typical verbs are 'einen Fond ansetzen' (to start/make a stock) or 'mit Fond ablöschen' (to deglaze with stock).
Der Braten wurde mit einem kräftigen Rinderfond abgelöscht.
The roast was deglazed with a rich beef stock.
The meaning 'Hintergrund' (background) is rare today. You find it almost only in art history or in descriptions of paintings and stage designs.
Das Porträt zeigt eine helle Figur vor einem dunklen Fond.
The portrait shows a bright figure against a dark background.
background
'der Hintergrund' and 'der Fond' can refer to the same area in images, on stages, or in scenes: the part that is farther back. 'Hintergrund' is the normal and very common word for this today. 'Fond' in this meaning sounds more technical, artistic, or old-fashioned, and is used much less often. The words can be confusing because 'Fond' really can mean 'background' in art. In everyday language, however, 'Hintergrund' is almost always more natural; otherwise, 'Fond' is more often heard for the rear area of a car or for a stock in cooking.

Der Maler setzte helle Farben in den Fond des Bildes.
The painter used light colors in the background of the picture.
Auf dem Foto sieht man im Hintergrund die Berge.
In the photo, you can see the mountains in the background.