I suspect that you aren't getting very good advice.

People rarely steal cars by judging the interior. They steal cars that they already know how to steal or smash every window of every car on the block hoping to find something left inside worth stealing.

Buy a club to put on your steering wheel and stop leaving stuff in your car if you're really that worried.

Make sure you understand what kind of insurance you have and get renter's insurance for your residence to cover theft of things other than your car.