My question is, is it okay to work in a car dealership seeing as most people buy on interest these days, and you might be involved in helping them buy?
My other question is, what is the ruling on working in big stores, as many stores like Walmart, Home Depot and Canadian Tire offer customers to buy on loan, thus dealing with interest. Since you will be working there as a cashier or salesperson, you might become a witness or assist them in the transaction, even if you are working in their corporate office. Is it okay as these companies are earning their profits through interest also?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
Please note that generally, it is not allowed for a person to undertake work which is Harām or work which involves assisting in Harām – the work has to be lawful and source of income or salary has to be Halal.
If the car dealership primarily deals with interest-based transactions and the majority of the employee's salaries are from interest-based income, then it will not be permissible for you to work in such a place.
In answer to your second question, there is no harm in your undertaking work in which you are not required to undertake any Harām tasks or directly assist in Harām activity or transaction. If the companies you mention have some interest-based dealings and your salary is a mixture of Harām and Halal income, as long as the majority of your income is from the Halal and not from Harām source it will be permissible for you to work in such companies.
Only Allah knows best
Written by Imam Abdul-Malik Sheikh
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Al-Fatawa Al-Hindiyyah Vol.5 Vol.343