What is this thing?

Just trace it out. It is likely some old alarm. You're probably going to find a black box zip tied under the dash somewhere. There will likely be a relay zip tied near this somewhere.

All aftermarket alarms are installed like this. It makes the car super easy to steal. The little relay is the ignition circuit for the car. The two larger wires on the relay is the main ignition wire the key switch activates. It would be much more difficult to find that wire without the black box and relay. When you get into the floorboard and really look under there, the aftermarket junk will be dead obvious.

The systems used to track a vehicle are nothing like this. There would not be a switch either. I have installed these tracking systems before as a side gig when some of my dealers didn't have much paint work for my auto body shop. The tracking alarm needs to be mounted just under the dash where the GPS and GSM antennas have as little metal between them and the sky as possible. The easiest way to do this on most cars is to remove the gauge panel. Smart dealers don't install the system in any obvious way or even use them to disable the vehicle. They just keep a spare key filed and repo the car if you don't pay, don't answer the phone, fail to call head, or act like an asshole about not paying.