He prays Zuhr in his car because he is the only Muslim in the factory

Praise be to Allah.

Firstly: 

Standing when offering an obligatory prayer is an essential part of the prayer and it is not permissible for anyone to pray sitting unless he is unable to stand, or standing will be very difficult for him, or he has a sickness which it is feared will be made worse if he prays standing. 

The basic principle concerning that is the report narrated by al-Bukhaari (1050) from ‘Imraan ibn Husayn (may Allah be pleased with him) who said: I had haemorrhoids so I asked the Prophet (blessings and peace of Allah be upon him) about prayer and he said: “Pray standing, and if you cannot then (pray) sitting, and if you cannot then (pray) lying on your side.” 

Ibn Qudaamah (may Allah have mercy on him) said: The scholars are unanimously agreed that if a person cannot stand, he may pray sitting. The Prophet (blessings and peace of Allah be upon him) said to ‘Imraan ibn Husayn: “Pray standing, and if you cannot then (pray) sitting, and if you cannot then (pray) lying on your side.” Narrated by al-Bukhaari and Abu Dawood, and by al-Nasaa’i who added: “If you cannot, then lying on your back; Allah burdens not a person beyond his scope (cf. al-Baqarah 2:285).” And Anas said: The Messenger of Allah (blessings and peace of Allah be upon him) fell from a horse and injured his right side. We entered upon him to visit him, and the time for prayer came. He prayed sitting, and we prayed behind him sitting. Agreed upon. 

If he can stand but is afraid that doing so would make his sickness worse or would delay his recovery or would be too difficult for him, then he may pray sitting. Something like this was stated by Maalik and Ishaaq. End quote from al-Mughni (1/443). 

Al-Nawawi (may Allah have mercy on him) said: The ummah is unanimously agreed that if a person is unable to stand during the obligatory prayer, he may pray sitting and he does not have to repeat it. Our companions said: His reward will be no less than his reward if he prayed standing, because he is excused. End quote from al-Majmoo‘ (4/201) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: The same applies to the one who is in a state of fear and cannot pray standing, such as if he is praying behind a wall and around him is an enemy watching out for him, and if he stood up he would appear from behind the wall but if he sits he will remain hidden from his enemy by the wall. In this case we say to him: Pray sitting. 

This is indicated by the verse in which Allah says (interpretation of the meaning): “And if you fear (an enemy), perform Salaah (pray) on foot or riding” [al-Baqarah 2:239]. So Allah waived the obligation to bow, prostrate and sit, and also to stand if he is in a state of fear. End quote from al-Sharh al-Mumti‘, 4/461. 

The fact that you are the only Muslim worker in this factory does not make it permissible for you to pray sitting in your car; rather you should pray in any clean place inside or outside the factory, offering the prayer in full and without rushing. 

It does not seem from your question that you are afraid of showing your Islam or praying openly; that is something very unlikely in your country. If we assume that a Muslim is living in a land where he cannot pray openly, then he must migrate from that land to a country where he may practice his religion openly. 

So fear Allah, may He be exalted, and try to perform your prayer as Allah has enjoined, and beware of neglecting it or being heedless with regard to its times and essential parts. 

We ask Allah to guide us and you and to make us steadfast. 

And Allah knows best.