Just a thought here... Why not just write a simple script that collects GPS location, host it on a private server, point an old Android phone at that page, and wire the phone under the dashboard?
That could work fine, but dedicated GPS tracker hardware isn't that expensive and will be more robust and reliable than an old phone (simpler software, heat and vibration tolerant) - plus there's usually IOT-NB/LTE Cat-1 support so you can use less expensive cellular providers and most trackers have at least one dry contact output for remote immobilization. The main downside to hardware GPS trackers is that they're mostly sold "to the trade," so you get a reference manual for their often AT-based communications protocol and you're left to your own devices. There are open-source backend servers around but I feel they're often too genericized to work well with the features of any one tracker, so big incentive to write your own software.
My boy had this in his integra ,it got broken into the other day, he has a kill switch on the ignition though, so the thief kinda just sat there in the car.
Kick your local thief in the face, ya?
It could be far simpler than that, effectively a stripped basic currently available phone wired into operating power that could be called to send GPS payload via SMS. It would simply power up on start and only ever send the coordinates based on an authenticated call or sms itself.