Inside the cabin.
A used seat belt is priced on 581 generations across 273 nameplates, listed below by make and model. Open one for the asking price on that car and what else the part fits.
Front seats and belts can carry side airbags and belt pretensioners. Leave any yellow connector alone if the yard has not made the car safe, and bring a second pair of hands for a seat.
Typical sizes, not a promise for every car. Everything to bring to a pull-your-own yard.