Here is your function updated. Tested and its working perfectly. Problem was that, it was passing value of first input box every time. I used $(this.element) to get current element on which autocomplete is requested. When dealing with classes we have to make use of (this) keyword to avoid conflicts.
function SearchText() {
$(".autosuggest").autocomplete({
select:function(event, ui){
window.location.href = '/Products/ProductInfoCenter.aspx?partnum=' + ui.item.value;
},
source: function (request, response) {
$.ajax({
type: "POST",
contentType: "application/json; charset=utf-8",
url: "/test.aspx/GetAutoCompleteData",
data: "{'PartNumber':'" + $(this.element).val() + "'}",
dataType: "json",
success: function (data) {
response(data.d);
},
error: function (result) {
alert(err.message);
}
});
}
});
}