Ok,
1. I do not know any dimensions.
2. I am unsure, do i take the blueprint/template and start mocking up the parts of the vehicle?
3. Do I create different artboards for each sides of the vehicle? (once i get the dimensions)
4. HOW to deliver the files, without the template or, the design on the template?
Or the set of artboards as mentionned in point 3 above.
Thanks
When you start with this, you will have to know dimensions. Period.
You will need to search for them, demand them from the client, whatever.
Contact the car manufacturer for a data sheet, search clip art companies (there are companies specialized in providing exactly this and you might need to bite the bullet and pay for it)
Place the blueprint in Illustrator and draw your design on it. You have made a layout or scribble already?
It makes sense to put this on different artboards. Maybe even different files. Depending on the size of the car you might need to work in scale.
Ask the printing company about #4. We can't read their mind unfortunately. Most probably they will need the template as well. But they will also have the car and will take additional measure directly from the vehicle and then compare with your file and maybe make adjustments. What they told you sounds like they don't want to talk to you. Unfortunately you need to cooperate with them. I don't know who is at fault for this communication failure, but you will need to fix it. You need their knowledge and experience. This forum cannot replace it.