You may replace either the entire lamp assembly or just the bulb.
If you choose to replace the entire lamp assembly, you will need to order part number 003-02956. Then, you should unsolder the two wires and unscrew the screw to remove the old lamp assembly. Reverse this process for installing the new lamp assembly.
Alternatively, after the old lamp assembly has been removed, you may just replace the bulb on this assembly (Bulb 1157 is available at most auto parts stores). Wearing gloves to protect your hands, pull the bulb out of the plastic tube. Install the new lamp bulb and seal it into position with RTV Silicone Rubber. Be sure that the seal is water tight. Put the lamp assembly back on the lid and screw and solder it into position.