I currently use GPS units from coverttrack.com. I have been using them for several years now for surveillance. There is a start up fee and then it is approximately $25.00 a month for month to month service. Im not sure you will find a free service out there!

Nelso Technology Pvt. Ltd. is best Indian manufacturer of GPS Vehicle Tracker.

Whatever you get, I definitely wouldn't place it under a seat. That would be too easy to disable / remove (the same thing goes with alarm brains. Too many people simply mount the brain under the dash. That's the first place a thief looks. Under the seat is probably second).

I would mount it behind an interior panel, such as in the cargo area. If it requires battery power, there's ALWAYS a way to hardwire power to it. You can even have a battery back-up for when the car battery is disconnected (DEI 520T battery back-up).

My $.02

And even then, success rate depends on how many squad cars in your local PD have the equipment installed and if they happen to be patrolling the right area.

any other alternative?

Get a ham radio technician license (a 6 year old can pass the test) and acquire an APRS device.

Study here:
https://hamstudy.org

Buy here:
Byonics - TinyTrak3 GPS Position Encoder

Track here:
aprs.fi

No monthly fee, ever and you learn a valuable skill.

I have a Vyncs car tracking device that pays an annual fee without a monthly fee. One thing that appeals to me is that it uses a 12V power adapter instead of a battery, and I don't need to worry about the lack of power. It has the functions of a normal GPS tracker, such as 60/30/15 seconds of update interval, with geofence and timely alarm function. In addition, this no monthly GPS vehicle tracker provides fault codes that inform the vehicle of fuel consumption reports, engine lights and other vehicle health information.

Check with your automobile insurance company - they may very well have a free GPS tracker that plugs into the OBD2 port in your vehicle that allows you (and them) to track your vehicle with an app on your phone. Easy to remove if the car thieves are smart enough to look for it - doubt it though

Google "Fleet Equipment GPS Trackers" - They are designed for Trailers/Equipment. Some of them have 3 years of service included. I have seen a few out there that claim up to 3 years of battery life. They use Cell/GPS, and you can set how often you want it relay tracking. You could probably even turn off tracking while you are actively driving the vehicle, camping, etc to save the battery even more.

I'm looking at something like this for my Overland trailer that sits in my driveway.

EQUIPMENT GPS TRACKER SYSTEM - GPS Tracking

A good poor man version is to use an Apple Air Tag.

The pro is if it’s stolen the chance on the thief having a iPhone is high which gives you a good chance to locate it. There are a few draw backs though. You’ll have to activate the find mode on the AirTag every time you leave the car. The life on the battery is about a year but constantly activating it will reduce it significantly, maybe every 6 months? Also, if the thief has a iPhone he will be alerted that a AirTag is following him. This could be a good thing. He may just abandon the car if he realizes he’s being tracked. Before you hide the airtag make sure you remove the speaker. There are many videos on YouTube that shows you how to do this.