What I do in these cases is to use a centre punch to put a dent in the metal where you want the hole.
Then on the other side you can see a small raised lump - the opposite of the dent.
The dent will be only 1 or 2 mm across and the same deep so if you decide not to drill there is no significant damage.
Of course this does only work reliably on metal. ...
Using a magnet would not work if the metal is magnetic (any steel or nickel alloy) as the magnetic force is more-or-less lost in the middle. I can understand how a motor vehicle would be challenging to determine a matching location.
A method that comes to mind is a length of transparent hose/tubing. An ordinary garden hose with suitable adapters at each end ...
I have a lot of vintage and antique appliances. When I want to strip then free of gunk I soak them in a pail with The Works toilet bowl cleaner which contains hydrochloric acid, then rinse VERY well, then dry immediately and oil lightly.
You can use vinegar but it takes longer.
That product is super cheap... a buck or two per bottle.
If you already have ...