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Here's a link to the diagram for a 2008 chassis, taken from the
Ford Body Builder Layout Book (PPLB) site. The trailer turn/stop lamps are fed from Fuse 26 in the Power Distribution Box, which is under the hood. It's a 20A fuse.
Sometimes the BBLB pages include a drawing of the PDB and the fuse box inside the coach. The pages for 2008 don't. So, you can download the owner's manual from
this Ford site. The fuse box info is on pages 194-200 of the 2008 E-450 manual.