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Okay, I'm running into a bit of trouble mapping my latest car. Here's a pic of it on paper.
http://i.imgur.com/xkTEdrW.jpg

I don't know exactly what's going on, but when I press "Unwrap", blender says that the mesh has non-uniform scaling, and that it will not work properly without scaling.

No idea what this means, but if it means that the mesh is not symmetric, that's impossible, as I either A: Edit symmetrically (on both sides of the car) or B: Cut the mesh in half, model it, then hit Shift+D and mirror the added faces, and join then. The mesh should be symmetric.

Any help on what this means?

Haven't used Blender much, but a quick search turned up this result:
Non-Uniform Scaling during UV unwrap. What is this?

Hope that helps.

Nice! Thank you! It seems I do remember scaling on the y axis (as a box, when I started the model). From now on i'll just move the faces where I need them, as scaling in the beginning seems to throw off the mapping.