[Plugin: Contact Form 7] Dynamic drop-down lists

Hi
I am trying to improve the contact form on my site so that when you choose some car maker on a drop-down menu, then the car model drop-down menu will change according to the maker.
for example:
drop down 1: Nissan, Chevrolet, Honda
drop down 2: please select maker
—
visitor clicked Nissan
—
drop down 2: 350z, gtr, maxima

Now, using javascript I was able to do that, but contact form 7 wont send the value with the email. this is weired but I am pretty sure it is simple to solve. I am not familiar with the plug-in code so that is why Im asking you help ??

here is my code:

<script type="text/javascript">
var maker = document.getElementById('maker');
maker.setAttribute('onchange','changemodel(this.value)')

function changemodel(maker) {
var model = document.getElementById('model');
model.options.length = 0;
model.options[0]=new Option("3", "3", true, false);
model.options[1]=new Option("6", "6", false, false);
model.options[2]=new Option("MPV", "MPV", false, false);
}
</script>

Thanks guys!