•1 Disconnect the ground cable by loosening the retaining bolt and pulling the clamp off the negative terminal. Disconnect the positive cable by loosening the retaining bolt and pulling the clamp off the positive terminal.
•2 Remove the battery from the vehicle.
•3 Label and disconnect the electrical leads for the blower motor. The blower motor will be located on the heater case which is located in the engine bay, bolted to the passenger side of the firewall.
Read more: How to Remove a Heater Core on the E350 | eHow.com http://www.ehow.com/how_5507454_remove-heater-core-e.html#ixzz1G93NkC6d