Cabin Fumes

[Forg]

To be honest, I doubt that you could have any fumes or gases made by the car's engine getting into the cabin & giving you a headache, without being able to smell it. The stuff coming out of an engine is pretty smelly, especially before the catalytic converter.

Total change-of-direction, but is there any chance it's had a windscreen change & the windscreen isn't perfect? I took my sister new car shopping a few years back & there was this batch of new Mazda2's that all had windscreens with an eye-height imperfection which caused that Carnival Mirror effect, and while it didn't bother me in the slightest (apart from being annoying given it was a new car & I therefore wouldn't buy one) it gave her a migraine.

 

[Poor old Dad]

Haven't run cabling for driving light switch (or whatever else) through a firewall grommet?

 

[Fu Manchu]

Getting headaches is a symptom of Carbon monoxide exposure.
You can get a simple CO detector for $30 or $40

We have one of these in the house and use it when the gas heater is on.
https://quell.com.au/product/carbon-monoxide-digital-display-alarm/
Would work just as easy in the car.

Also steam cleaning the interior won't remove VOCs. They are just there because of the materials used in manufacture of the car.