"Where is the customs?"

Translation: 海关在哪儿？

[PeaceJoyPancakes]

"Where is customs" is better than "where is the customs".

[ArtBurnap]

I agree. At least in my type of American English, I don't think I would ever use 'the' with the word 'customs' (in the sense of 海关), unless 'customs' were used adjectivally to modify another noun, as in 'the customs office / officer.'

[PeaceJoyPancakes]

Yes, I agree with your expansion of the point.