How could people in the medieval times defeat an armored vehicle?
In my setting there is a mad artificer making armored cars for the army. The cars have wooden frames with 10 mm of steel on every side, wooden wheels with metal plates covering them, and are propelled using two different methods (some have horses which sit within the cart and push it forward, and the others have slaves inside who use large handcranks to rotate the wheels). They have several wheels to get even ground pressure and all the openings (two vision slits, and multiple arrow slits around the vehicle) are covered with visors that can be closed. Some of the larger cars also have cannons mounted in them, which can fire forward, and others have small cannons (basically just oversized muskets) or magic machine guns mounted in a turret.
The infantry fighting the cars would need methods to face the cars both in open combat (meeting on the field of battle) in random encounters (infantry have no time to prepare) and ambushes (where the infantry have time to prepare)
With this information, how could people in a low fantasy setting with 1400’s technology and limited magic defeat these vehicles?
Edit: After reading the input I have decided to adapt the concept for the cars. 46
Lots of people have mentioned mud being a problem, and I do not know a fix for that besides wider tires and each wheel having it's own suspension so that they all touch the ground (and perhaps more wheels).
People have also mentioned the use of artillery against the car, and while that would be a threat I don't think that the weapons which would be used in this time period would be good enough to reliably take out the cars.
Something to mention that I should have before, the setting is very low magic. Someone having magic ability at all is one in a million, and the odds of that person also being skilled and smart enough to become an artificer would be incredibly rare. As far as the setting is concerned the artificer is the only person capable of making any sort of advanced weapon (like the magic machine gun).
Otherwise some of the answers have been very useful. The use of Naphtha (something I didn't realize was known about at the time) is especially interesting. 523