What is this?

Ok. Under the dash, right at drivers side by radio, there's a little stainless cylinder. About 1.5 inch long. Definitely the highlighted component in the schematic picture attached. What is it? I understand is in line with the interior lights. It stays physically hot when key off, have a ground/short issue causing no turn lights....not sure if they're related.

For those who enjoy a puzzle (know I haven't given up yet): All other exterior lights work including flashers, brake, trail and headlights. Both flasher units work because they both power the flashers. Power is getting to turn signal flasher unit and turn signal switch. I will ohm turn signal switch tonight, but don't think that's the issue. All bulbs ohm'd out along with grounds cleaned and checked from engine/battery, battery/radiator support, rear harness/exposed steel in trunk....

Any help appreciated.

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