A huge amount of impure stagnant water from a pit on the road splashed inside my car, leaving the car upholstery wet, now what am I to do given that the upholstery inside the car cannot be washed as it would damage other stuff in the car, so how am I to purify it?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Firstly, it should be underlined that it is difficult to judge that this kind of water is ritually impure. The Maaliki scholar Al-Mawwaaq wrote:

"Al-Mudawwanah reads, 'There is no harm in the mud on the roads and pathways caused by rain if it gets onto one's clothes, body, leather socks, and shoes, even if it contained dung or other impurities. People walked on such muddy roads and performed the prayer without washing the mud off...' The Shaykh (Ibn Abi Zayd) said, 'This is unless the impurities are more than the mud or the impurity itself is still present (and identifiable).' Ibn Basheer said, 'The statement may indicate conditioning the ruling to the situation when the impurities are more than the mud on the pathways. It could also indicate that even if the impurities are more than the mud but the person has to wear the mud-stained shoes or clothes, it is not obligatory to wash them, as is the case with the clothes of the wet-nurse.' Al-Baaji wrote, 'The impurities that splash from the roads are excused as long as the essence of the impurity is not seen and the person believes that it is not most likely present.'" [At-Taaj wa Al-Ikleel]

Assuming that that stagnant water is impure and is not excused and it stains something that is damaged by washing with water, then it is to be purified by other means that would remove the impurity without causing damage. Shaykhul-Islam Ibn Taymiyyah wrote, "The impurity may be purified with any pure fluid that would remove it, such as vinegar and the like. This was one of the views of Imaam Ahmad, and it was favored by Ibn ʻAqeel, and it is also the view of the Hanafis. If the impurity got onto something that is damaged by washing it, such as a silk garment, paper, and the like, then it is enough to wipe it, according to the more correct of the two scholarly opinions in this regard." [Al-Fataawa Al-Kubra]

Allah knows best.