One possibility the reason that this AirTag is not making a sound, is that the individual removed, or otherwise, disabled the tag's speaker prior to placing it in your car. As such, any attempt to have it make a sound would be thwarted.
If it were me, the following is a list of possible places I'd search on your vehicle, to hopefully, locate the tag. Note that some of these places may require a trusted mechanic or friend to assist you. You may have already searched a number of these.
Inside the passenger compartment:
- Under the driver's and passenger's seats, especially toward the rear and underneath the seat rails
- Under/behind the rear seats and inside gaps where the seat backs meet the cushions
- Inside seat-back pockets
- Under floor mats and carpeting, including edges that can be lifted
- In the trunk/cargo area's side compartments
- Inside the spare-tire well or storage compartment
- Behind removable trim panels in the trunk
- Inside the center console and its storage compartments
- Glove compartment, particularly behind/underneath it
- Door pockets and the spaces immediately behind interior door panels
- Under the dashboard, especially around the lower dash/footwell areas
Exterior — this is particularly important:
Use a flashlight and, if possible, a mirror or your phone camera to inspect:
- Behind/inside the front and rear bumpers
- Inside wheel wells, particularly behind the plastic liner
- Behind splash guards
- Under the vehicle's rocker panels/side skirts
- Along the underside of the vehicle
- Around suspension components and frame members
- Behind/near the spare tire from underneath
- Around trailer hitches
- Inside/behind areas where plastic underbody panels meet
- Anywhere a small object could be attached with a magnet or adhesive