[Rise]

Kolko, так еще можно:

<select id="marka" onchange="changeSel()">
	<option value="">----------</option>
	<option value="Mersedes">Mersedes</option>
</select>
<select id="model" onchange="changeSel()">
	<option value="">----------</option>
	<option value="S-Class">S-Class</option>
	<option value="Sprinter">Sprinter</option>
</select>
</br></br>
<input type="text" id="auto">

<script>
function changeSel() {
	if (marka.value) {
		auto.value = marka.value + ' ' + model.value;
	} else {
		model.selectedIndex = 0;
		auto.value = '';
	}
}
</script>