Get back an uninsured car

The tracker puts a marker on the map, similar to the icon used for the Impound in offline player.

I believe the marker is called "Personal Vehicle" on the legend, so you should be able to find it fairly quickly.

Alternatively, if the marker isn't visible on the map, use your phone to call the insurance company and request a replacement.

If your car is not insured you can simply destroy it and you will be rid of it.

For insured cars it is a bit trickier:

• If your car is worth less than $50k you will be able to sell it to Los Santos Customs and get rid of it permanently that way.
  
• If the car exceeds that worth you can replace it with another car when your garage is filled completely. Just drive a car into your full garage and you will get the warning that the garage is full and if you wish to replace a car. After the loading screen you will be prompted to choose a car for replacement.
  Be careful with pressing the "OK"-button as to not accidentally lose the first car on the list, if it is not the one you want replaced.