You should be able to upload now.
Well… my question would be what is the purpose of using Seamly for the seat covers? Would it be a one pattern deal? Or is the idea to take a basic seat pattern and make variations on it? Is it to make patterns available to others? If the purpose is to just make 1 set of seat covers there are methods more suitable than Seamly. Also Seamly’s main advantage is drafting one pattern and having it resize to many people’s measurents. I don’t think resizing car seats is a needed thing?
That being said… since there are no “car seat drafing systems” what I would do is drape a seat with cheap muslin to get patterns of all the pieces… then use a radial projection method (or variation thereof) to “digitize” to Seamly. If you picture the muslin pattern as a grid, it’s a matter of transferring all the grade or important points on the muslin to Seamly in an X-Y space. I actually outlined a few years back a couple patterns where I did exactly this… one was a draped shift dress the other was a pre-existng mens jacket.
You might even be able to use the image import feature now to draw over photos of the muslin pieces.