besides what ratman said, there's also a hidden screw that holds down the DM1. You have to remove the panel that covers the rca inputs as well, and is held on by three tiny hex screws.
Once you have the back and this cover off, carefully disconnect the three plugs that go into the power supply board, they're white and have side clips that secure them. Also unplug the coax that goes to the directTV card reader.
The DM1 has two brackets that slide into the plastic base that holds the fan. Tilt the DM1 towards the picture tube and slide back. You'll see the fan, and can remove the base (two screws) to clean and lube.