<form method="get" action="db.php">
<dl>
    <dt>
        Part:
    </dt>
    <dd>
        <input type="text" name="part"></input>
    </dd>
....
....
<a href="#" onClick="document.getElementById('part').value = 'motor';">motor</a>

TypeError: document.getElementById(...) is null

<a href="#" onClick="document.getElementsByName('part')[0].value = 'motor';">motor</a>

<input type="text" name="part" id="part"></input>