the PIR sensor is what you use in automated lights at entrance of garage for example. Nowadays you can buy such lighting with the sensor embedded in the design of the lighting fixture like this image
the sensor could detects any presence/movement in the full volume of your car in one go if you position it "well" and will require less drain on your car battery than 15 laser that you need to maintain fully powered.
May be you can read this howtomechatronics.com article (where the image above comes from)
(also your laser would have to be very well attached to not move by 1mm which could happen when you drive your Jeep on bumpy road. any misalignment with the sensor on the other side would lead to triggering the alarm immediately when you turn it on... seems high maintenance to me.)