Uncheck all checkboxes in a GridView on a button click using client side script

I have a Popup which contains a Panel with GridView, 3 buttons inside it.
When i click on Uncheck All button i want all the checkboxes inside the Gridview to be unchecked. Below is my code. When click on Uncheck All button, checkboxes are not getting unchecked. 

<asp:Panel ID="pnlVehicleSelection" runat="server" Style="display: none" CssClass="pnl"> <div><br /><br /><br /> <asp:Label ID="Label6" runat="server" class="formlabel2" style="margin-left:5px;margin-right:25px;" Text="Select Year Make Model below:"> </asp:Label> <asp:Button ID="btnSaveVehicleSelection" Text="Save and Close" runat="server" AutoPostBack="true" OnClientClick="$find('mpexVehiclepopup').hide();"  /> <asp:Button ID="btnCancelvehicleSelection" runat=&q ...