Customer Account in Magento: Set Customer Account Link Active (even on sub pages)

layout.xml

To manually specify which tab that you would like highlighted in the customer 
account section, you will need to specify the setActive method with reference 
to the customer_account_navigation block.

<?xml version="1.0"?>
<layout version="0.1.0">
    <dealers_orders_orderdetails>
        <!--HERE-->
        <reference name="customer_account_navigation">
            <action method="setActive"><path>dealers/orders/</path></action>
        </reference>
    </dealers_orders_orderdetails>
</layout>