Cleaning, cleaning, cleaning.
I picked up an 08 Sport V8 model that joins my wife's 5th gen in the garage. It's in great shape overall and appeared to be in great shape inside until I started looking under things and found enough hair to make at least three more dogs.
After pulling out the seats, console, and all the plastic trim I *think* I have the majority of it clean. It didn't really smell like a dog at all (the PO did a good surface detail) but I have my ozone generator running in it now to take care of any smells and it should be good to go. That was a lot of vacuuming...phew.
I took apart my interior as well. My 03 is in great mechanical shape but the interior was not taken care of. I took out seats, console, carpet, door panels, rear panels, and rear parcel shelf. Super easy to clean carpets out of car. I vacuumed and wiped down everything including floor board and inside the doors. I cleaned every part that came off the car. I had the dash apart for a noisy HVAC actuator and instrument cluster repair and cleaned as much as I could get to inside the dash. 20 years of grime and muck gone.