If we're speaking only about pup safety and not cargo concerns I use the Ruffwear Load-up Harness as it is specifically designed and tested to be a seatbelt for dogs. By tested I mean they actually perform crash tests with dog-dummies! To my knowledge there are two companies who make crash tested seatbelts for dogs, The
Ruffwear and the
Sleepypod. You slip the standard seatbelt through the loop, tighten it all down, and they are good to go. I use mine in combination with a Dog Hammock which is basically a U-shaped second-row cover that keeps the dog in a nice little pocket.
Here's one example that also has door covers, which I may buy to replace my current model since the dog can still accidentally damage the door panels while shifting in turns. You can buy the ones without door protectors at discount stores like Ross, Marshalls, etc., also or just find on
Amazon. Be sure to get one with some padding underneath, non slip ideally do pooch isn't sliding all around, and water proof for spills or if they get car sick.
For dogs placed in the car area you can use the dog sehtbets above and clip them in place to the floor mount D-rings which would limit movement somewhat to keep them safer but not fully so they can still move around. I do refer them being in the second row just so I can keep an eye on them and make sure they aren't twisted or caught up in the seatbelts. The belts tend to work better with broad-chested dogs and can move around a bit on narrow-chested dogs.
I just lift my 12-year old, 80-lb Pit Bull/Bulldog mix up and into the car, she can jump out on her own however. For a cargo area liner I use the Weathertech which some people don't like but works great for me. No it doesn't have 100% coverage but that's because it allows for the third row seats to still be used. I cut out the areas where the seats mount and put the OEM plastic bezels back on over the weather tech liner to hold it all in place and so I can use my third row seats without removing the liner.