Why isnt this working?

[wxdqz]

function alterDrop()
{
var d = document;
refPage = d.referrer.substr(d.referrer.lastIndexOf('/')+1);
if (refPage = "page1.htm")
{
d.form.enquiries.requiredsubject.value(page1);
}
else if (refPage = "page2.htm")
{
d.form.enquiries.requiredsubject.value(page2);
}
}

This is my code inside my SCRIPT tags inside the head. I have also put the onLoad="alterDrop();" in my BODY tags.

My aim is to make the dropdown box in my form auto select an option when the page opens, dependent on the referring page.

But it doesnt seem to be working... help me? please ???