The way I estimate this is just assume about 2 tons of CO2 per ton of steel[1]. Based on that it's hard to see a normal car requiring more than 5-6 tons of CO2 to produce.
[1] Chemical reduction dominates everything. The energy required for forming and assembly would be much less.
Some metals can undergo a bunch of reheats in processing (3D printing of metals is particularly bad) which could approach chemical reduction energy, but you're right to zeroth order (which is good enough in these conversations, IMHO).
One other point is most automobiles are now recycled. Going what one reads on the internet about 25% of the steel in cars comes from recycled steel. And 95% of cars are fed into the recycling stream when they are scraped.
[1] Chemical reduction dominates everything. The energy required for forming and assembly would be much less.