Your example:
$WADbSearch1->addComparison("IDModel_new","".((isset($_POST["model"]))?$_POST["model"]:"") ."","AND","=",1);
$WADbSearch1->addComparison("FirstYear_new","".((isset($_POST["year"]))?$_POST["year"]:"") ."","AND",">=",1);
$WADbSearch1->addComparison("LastYear_new","".((isset($_POST["year"]))?$_POST["year"]:"") ."","AND","<=",1);
would create the query:
WHERE IDModel_new = 'Mustang GT' AND FirstYear_new >= '2000' AND LastYear_new <= '2000'
so you see...
Even though you want
"Greater than or Equal to >= First Year
Less than or Equal to <= Last Year"
since the column is actually on the left hand side of the comparison you are specifying:
"First Year >= the entered value
Last Year <= the entered value"
Which is incorrect, and which is why my suggestion should be correct.