Is this safe to do? Will leaving the windows open a crack be sufficient? Are there specific target temperatures for feasibility?
This is just anecdotal evidece but when we were camping near Prague in summer, we would dry our towels after swimming by just hanging them over the seats in the car.
I don't remember doing anything extra like leaving the windows open. It was a small car (Fiat Panda old model), the temperatures were around 25-30 degrees Celcius. Bear in mind it was only 2 not-so-wet towels and maybe the occasional socks/underwear, and the humidity outside was not very high (ie continental climate).
In summer, we cannot run blower in a car. We should hang the clothes near the windows of the car and park the car in the area where sunlight falls directly on the windows of the car. We can keep the car windows opened to some extent keeping in view the security aspect. If the car is running, we should hang the clothes near the windows and keep the windows open if air conditioner is not running.
In summertime you can definitely dry wet stuff in a vehicle. A vehicle with the windows up the interior easily reaches 50+ Celsius (122+ F). I have measured temperatures inside a vehicle as hot as 73 Celsius (165 F) with a infrared thermometer which is a hot as a typical clothes dryer.. If anything it would dry your clothes so quickly that would become stiff from unwashed dirt.
Another alternative is using the air conditioning as a dehumidifier.. takes longer but will work as well.