1. Made a chain that fits over my gear shift twice (hold the ends together to make a loop, slide over the shift, twist, slide over again - if there's some wiggle room, you're at the right length!).
2. Slip-stitch the end to the start to make a loop, then single crochet all around. Cut yarn and finish off.
To use, slip the loop over your gear shift. Slide the bags onto the loop (you need the kind of reusable bag that has a little loop for hanging). Loop the loop (haha) back over the gear shift. Secured!
Hope this is a helpful addition to your car!
UPDATE: It's not just for bags! This pattern is what I used to make bootloops / bootstraps for my ski boots and snowboard boots. You loop the loop through the loop on the top back of the boot to hold the boots together for ease of carrying. You can make the loop a little thicker for this (add another round or two of SC) so it's more comfy in your hand, since ski/board boots are not light.