Since Q1 2011, the RadExpander's Template was re-factored and its size was significantly simplified. Actually, there is no DecoratorTemplate anymore although the DecoratorTemplate property still exists.
I created a sample project where I edited the default template of the RadExpander from Q2 2011, commented the parts that define and control the Expander Button and used your XAML that creates the "+" -button ( the two crossed Rectangles. Hope you will find it useful.
the Telerik team
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