All I can say is no wonder it took you a while to get the parenthesizing right when you've formatted (or haven't formatted) the query like that. What's wrong with some return characters in there?

my $sql="
SELECT
        idNum,
        (First(price)-Last(price)) AS difference,
        Min(dateCreated) AS minDate,
        Max(dateCreated) AS maxdate,
        Last(mobile) AS lastMobile,
        Last(contents) AS lastContents,
        (( (First(price)-Last(price)) /First(price) )*100) AS percentC
+hange,
        Last(price) AS lastPrice,
        Last(state) AS lastState,
        Last(contact),
        Last(year)
FROM carAdds
GROUP BY idNum
HAVING (( Last([year]))>=$yearmodel)
ORDER BY (( (First(price)-Last(price)) /First(price) )*100) DESC
";
[download]

Doesn't that look better? It'll be much easier to debug and change, and understand for the next guy, too.