Source of irritating motoring directions (8)

I believe the answer is:

backseat

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'source of irritating motoring directions' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'source of' is the wordplay.
I cannot really understand how this works, but
'source' could be 'seat' (I've seen this in another clue) and 'seat' is located in the answer.
The remaining letters 'back' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for backseat that I've seen before include "Secondary or inferior status" , "driving position - that's critical" .)

I've seen this clue in The Irish Times.

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