The vehicle isn't perfectly sealed and air is able to readily migrate from inside to outside, so the relative humidity will change with weather. Changing temperature will also change relative humidity as RH is a measure of the humidity in the air relative to the maximum it can hold. Colder air can hold less moisture than hot, so as the air temperature drops the RH will rise but the actual amount of moisture in the air is exactly the same. Then there is dewpoint to contend with, but that's a different problem.
The smell is most likely your AC condenser inside the car as these end wet when they cool the air below it's dewpoint, and then become a breeding ground for bacteria that smells. Get a "bomb" to deal with it, as has been suggested, as they are inexpensive and affective. There should be absolutely no need to run a dehumidifier in a vehicle that is in regular use as it is getting heated up when used and there are huge air changes forced by the interior fan. If that isn't keeping moisture at bay then there is likely a leak that needs finding and fixing. A vehicle parked up outside in winter and out of use would probably benefit from something to deal prevent problems caused by moisture, but that could just be moisture absorbing crystals.
I've used this in a work van that smelt like a pack of wet dogs and it worked wonders https://www.halfords.com/motoring/engine-oils-and-fluids/air-con/a%2Fc-pro-auto-air-con-cleaner-538198.html