Okay, okay. First of all.. you're trying to get her used to walking outside on the leash AND get her used to car rides, right?
You need to separate the two. She may be fearful of going outside on the leash BECAUSE she sometimes goes on a car ride when the harness and leash go on. I say don't expect too much of her too fast. Stop trying to get her used to car rides for a little while and focus on walking outside.
Are you familiar with positive reinforcement? Have you ever used clicker training? It can do wonders for fearful or shy dogs.
Even if you're not, I recommend buying some deli meat. Something plain (like low sodium turkey breast--nothing too fancy or heavily seasoned). Shred it into tiny pieces about the size of your fingernail or smaller and put them in a ziploc bag. Keep it in the refrigerator and every time you want to take her outside, go grab the bag and tell her, "Outside!" in a happy tone (or "Do you wanna go outside?" Whatever you choose, be consistent.) Drop a few treats on the ground for her to eat while you attach her leash. The idea is to make attaching the leash and going outside on walks a positive experience, so do this every single time.
The biggest mistake you can make is to coddle fearful behavior. Don't talk to her in a "Oh you poor baby" voice. Don't talk sternly either, of course! Just talk quietly and matter-of-factly. Be encouraging and optimistic. Be her pillar of strength and show her that the world isn't as scary as she thinks. Never drag her on the leash.
Every step she takes should be of her own free will, and her progress will be rewarded by you with praise and treats.
If she is scared of a particular inanimate object outside, go up to it yourself. Touch it, knock on it, pick it up slowly.. whatever you can do to show her that it's not so scary. (Keep her away from people or dogs that would probably cause her to have a scary experience though.)
IF YOU'RE NERVOUS ABOUT HER BEING ANXIOUS OUTSIDE, SHE
WILL BE ANXIOUS OUTSIDE.

Dogs are very keenly aware of our emotional states, even when we ourselves aren't aware of how we feel. That leash acts like a telegraph wire-- if you're nervous, you will tense the leash without thinking about it, and that translates all the way down to your dog who then thinks, "Mom is nervous.. I bet I should be nervous right about now, too."
The bottom line is that if during one session all you can get her to do is leave the front door and stand cowering on the front step for ten minutes, then so be it. Let her do that. I promise you she won't do that forever. Avoid eye contact with her and don't talk to her too much while she's nervous, but JUMP on an opportunity to praise her and give her tons of treats if and when she decides to take that first step off the doorstep.
Also, be aware of your tone of voice. You shouldn't sound like, "Come on. Now we are going to train you to walk appropriately on the leash.

" Instead, try to sound like, "Aren't you happy about this opportunity to earn lots of tasty treats and see the exciting world outside?!

"