Forgive me if I've got it wrong as I haven't followed this case from the beginning, but EA's car was impounded a day after Christina's disappearance, am I right?
I couldn't help but notice the bolded bits in the article marking her 1st missing anniversary.
Those same chest-high weeds and trees would also have been blooming when Christina disappeared. They'll be different in specific locations all over Texas because plantlife and pollen is so varied. The (relatively) few experts could tell where any samples came from, though it might take them a while.
Anybody who locked Christina in the trunk of a vehicle and then maybe took her into woods or a natural area and disposed of her, would have brought a lot of plantlife back into the driver's seat of the car. Pollen, seeds, leaves and spores wouldn't easily be removed by normal cleaning. And with a little luck, a sample of a species really unique to a small location could be in the car.
Hope investigators have invested in the services of a palynologist. Don't think they have though.
This website has good info on the kind of services that can be provided, though it's in New Zealand.
http://www.theforensicgroup.co.nz/expertise/pollen/