In my rig, that is the battery vent. Expanded metal or screening may keep them critters out if a "NO MICE WELCOME" sign is ignored. I have never had any mice in mine.
2011 351RLTS Eagle, Mor/Ryde suspension & pin box
06 F350 Lariat PSD, SRW, LB, CC, EGR delete kit, 16K DrawTite hitch, Timbrens, TST TPMS
2017 GMC Terrain (future TOAD)
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