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Remove the headlight on that side and open the rear cover. You most likely have wiring inside the headlight that has cracked and missing insulation, causing a short.

If the wiring looks fine (don’t forget to remove the connector from the headlight and check the wiring on the back of it), remove all light fixtures on that side. Replace them one at a time until the fuse blows.
If it happens with the headlight or taillight, remove all bulbs and replace them one at a time until the fuse blows.