I don’t think the location is ever specified, beyond it being on his forehead.
When it’s first introduced in Philosopher’s Stone, Harry is just a baby, and all we know is that it’s on his forehead, behind some hair.
Under a tuft of jet-black hair over his forehead they could see a curiously shaped cut, like a bolt of lightning.
It gets a few other mentions throughout the book, where it’s consistently described as “on his forehead”, but with no further detail. For example:
The only thing Harry liked about his own appearance was a very thin scar on his forehead which was shaped like a bolt of lightning. […]
Mr Ollivander touched the lightning scar on Harry’s forehead with a long, white finger.
Most of the later books make reference to it near the beginning, to refresh the reader’s memory. None that I can find refer to the position on his forehead. Here are a few examples:
In Chamber of Secrets:
He wore round glasses, and on his forehead was a thin, lightning-shaped scar.
In Goblet of Fire, a chapter called “The Scar”:
The old scar on his forehead, which was shaped like a bolt of lightning
Order of the Phoenix:
the old scar on his forehead prickled uncomfortably
Or Half-Blood Prince:
“Oho,” he [Slughorn] said, his large round eyes flying to Harry’s forehead and the lightning-shaped scar it bore. “Oho!”
All the way to the end of Deathly Hallows:
He lowered his hand absentmindedly and touched the lightning scar on his forehead.
None of these quotes make any reference to the location of the scar, and there are others like them throughout the book. This isn’t a proof that it isn’t described in the books somewhere, but most descriptions seem to omit it, so I don’t think it’s there.
(Which is not to say JKR did not have an idea of where it was – as the quote in user13267’s answer shows, she had some thoughts on it – merely that it doesn’t show up in the books.)