I looked out of the window and saw his car down at/in the parking lot.
anonymous I looked out ( of ) the window and saw his car down in the parking lot. 'of' is optional. I'd omit it.
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anonymousI looked out (of) the window and saw his car down in the parking lot.
'of' is optional. I'd omit it. And don't use 'down' unless you are looking at the parking lot from a height.
The word you want is 'in', not 'at