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

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.