If, as I suggest, you are sleeping in a car with a car cover, your problems don't end with staying warm at night. On most days the sun will enforce a wake up time. Cars heat up. On sunny days in California, sleeping past 10:00am will make you sympathize with baked potatoes, so get up before you get cooked. Also try to park under shade trees or next to a building that will cast a morning shadow on you.
During really hot times of the year, under blistering sun, even getting out of the car and finding shade may not be enough. At those times, exploit the commercial sector. Malls are a great place to hang out. Loitering is not prohibited in malls, so long as you don't look like a stereotypical homeless person. Go in dressed well. Bring your hygiene supplies and get cleaned up in one of the mall bathrooms. You may as well solve two problems at once. Spend the day window shopping and eating samples at the food court. On a hot day, when you are feeling idle, you could do worse.
Go to a movie matinee. It's the cheapest time to see a movie, and it is in the hottest time of the day, between noon and 4:00pm. Movie theaters are air conditioned, restful environments and if you aren't interested in the movie, no one will notice you snoozing unless you snore. Even then, many matinees run without an audience. You may have the entire theater to yourself.
McDonald's, Jack in the Box, Carl's Jr., Burger King, Wendy's, KFC, Church's, Taco Bell, these places have two things in common, bad food and air conditioning. Buy a soda and you can sit and nurse it for several hours. Often the refills are free as long as you are in the dining room. This will keep you out of the sun in the afternoon.
Wear sunscreen. Seriously, wear it. Sunburn is a terrible hazard to the homeless. You can get a sunburn even on overcast days, since the radiation that burns isn't even slowed down by cloud cover. Chronic sunburns can lead to open sores and infections. It is thought that skin cancer can be caused by only a few serious sunburns, and may show up decades later, so stay safe. A sunburn is a tipoff that you are homeless, in addition to being a health hazard. During the summer, wear sunscreen or keep out of the sun.
If it is really hot, dress appropriately. Don't try to carry everything you own. The car is a great storage bay. Dress lightly, but bring a windbreaker. When you go into the movie theater, restaurant, or mall it may be too cold for comfort. People overcompensate.