Enable - Disable <select>.

I've this code:

function checkdate(FormSubmit){
alert(document.getElementById('Mois').value);
if(eval(document.getElementById('Mois'))>0 &&
eval(document.getElementById('Annee'))>0){
document.forms['FormSubmit'].getElementByID('SELECTCONSTRUCTOR').disabled
= false; //can't reach the control here
}
else
{
document.forms['FormSubmit'].getElementByID('SELECTCONSTRUCTOR').disabled
= true; //can't reach the control here
}
}
....

<form name="FormSubmit" method="post" action="">
....
<select name="Mois" onChange=checkdate(this.form);>
....
<select name="Annee" onChange=checkdate(this.form);>
....
<select name="SELECTCONSTRUCTOR" disabled>

Now, what I would like, is to enable the SELECTCONSTRUCTOR if month and year
are selected (I mean different to 0).

I can't enable or disable. In fact I can't reach the control in the
checkdate function...

How can I enable/disable SELECTCONSTRUCTOR when my 2 fields are selected?