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Enter the The Thorn (at least, before midnight and after 6:00 am) and head south to find an area.
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The Westside West Entrance is, of course, to the west, but south-east of there is the Westside South Entrance, near which is a manhole cover leading to The Thorn, which also has its own map marker. There are two entrances to Westside, as was pointed out earlier during Contreras’s mission.
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First, however, it’s time for a little detour which will prevent you from having to backtrack later, made more palatable by the fact that Westside - the detour’s location - is also fairly close to Vault 22.
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