The mean incubation period is 4-5 days. It is thought at most to be 14 days. It becomes increasingly unlikely that you have it after 5 days without symptoms.
Do you know how your cousin's death was attributed to Covid, if he was asymptomatic when you saw him approximately eight days before his death, as per your timeline? Was an immediate cause of death given? (i.e. Covid can cause death by a few different means--pneumonia and clotting, to name two). It may be possible that your cousin was an asymptomatic carrier, and his death was due to a cause with no outward signs prior to death, like a clot, if you do not know of him having the classic Covid symptoms of high fever, coughing, and difficulty breathing. This is rare, though. A negative test does not prove that someone is not infected with the virus, either.
Being in a car with a mask on when the other person is not is still a high-risk scenario for the mask wearer. The ability of a mask to protect someone is dependent on the type and quality of the mask used. With most common mask styles worn by the public, mask use is primarily about the protection of others, by containing the respiratory droplets of those who might be infected.
A car (presumably with windows up at this time of year) is a fairly poorly ventilated space that puts people in very close proximity to one another. It is now thought that Covid spreads significantly due to the aerosol route, and not purely from projected droplets, although droplets are still thought to be the primary route of infection.
Droplets behave ballistically, in that they move largely under the influence of gravity and travel a short distance before falling to the ground. Aerosols are smaller, and float on the air, and are not so easily contained by masks. They travel further and remain airborne longer. Spending time with someone in a small unventilated space therefore is a significant risk, even if one party is wearing a mask.
I think that mask wearing is sufficient as long as you are spending a short time in the place where you are wearing a mask, everyone else present is also wearing a good-quality mask in that place (no exceptions), and that space is also well-ventilated. If an indoor situation falls short in any of these regards, I would not enter even when wearing a mask. At least, not until this pandemic is over.