Big t, when you attempted to have your cat in the vehicle, did you bring some dirty litter several deposits of both? Sounds counter productive but cats want their scent on everything from bedding to potty.
BTW hydrogen peroxide ( I use it straight on the walls of my litter boxes...got one boy that wasn't taught to squat pee) can help neutralize cat urine smell. I have one btl outside my front door as a roving unaltered Tom seems to like marking my storm door...guess he smells my boys and girls. But use a test spot on fabric to see if it will alter the color....ask me how I know that lol!
When you tried before to take your kitty did you let her investigate the vehicle? Without it being on or moving...Again smells are comfy, toss a few small towels she's slept on around. Her food water bowls, toy or two if she has favorite and her bed ( if she's anything like mine she'll find her own napping spots.
Here's steps: for some it's a process, for others they are just switch key cats.
Quick method
Day one morning: Introduce cat to vehicle (with her in kitty carrier getting used to being loaded up but not going to vet) with pm let her explore start with 15 min exposures . First 5 mins with door closed and carrier covered ( dark place safe place)
Have a litter box set up ( you can buy a cheap plastic container with a lid from Walmart and canit store in garage outside after training session)with unused litter on bottom and the used pile on top evening ( another quick get to know her new home session) feed her ( special little treat so she learns vehicle equal tasty tidbit...may be excessive but you want her co-operation... two morning and two night sessions. Lots of reassuring petting and a special treat for being a good girl. I have food motivated cats so I use canned chicken as treat.
Day three 30 minutes in vehicle ( starting it so she becomes accustomed to sound and vibration about the last five minutes), lots of petting, talking to, etc. New surroundings lots of things to see hear smell. More nummy treats as reward for being such a good girl.
Day five load her up drive around the block. Several times a day (Ideally she would learn that getting into the carrier means if she goes in willingly... she gets treat, I have two that will volunteer get into kennel when I get it out, one hides and cries like he's being tortured) mine has a special blanket and riding harness for riding copilot in passenger seat.
condensed version to get cat training for travel.