I HIGHLY recommend a back seat "hammock" to contain all of the doggie mess. I've had two hammocks from other cheaper brands before but they started falling apart quicker than I expected. I decided it was worth buying a more quality one so I got this one from Orvis:
Windowed Dog Hammock Car Seat Protector / Grip-Tight Windowed Hammock Seat Protector -- Orvis
They have other ones available as well. It is holding up extremely well against our two dogs, a Yellow English Lab and a Catahoula. Zero tears, doesn't smell, washable and easily installed/removed. I have an awesome dog bed that I got from Orvis as well. Just real high quality stuff from them, never been disappointed.
For those that have a dog that sheds a ridiculous amount (like my Lab) I'd recommend using a furminator desheding brush at least once a week to cut down on the amount of hair that gets deposited in the Model 3.
Oh, and be prepared to vacuum your Model 3 at least every other week. And keep some glass cleaner and a microfiber towel handy to take care of all of the nose prints on the windows.
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