Left directly in the sun? Not a good idea especially in a hot country. Not just heat but UV damage will cause it to deteriorate.
If it must be left on top put it in a white, non transparent Pi box. White? To deflect heat because darker material will absorb heat and you want to avoid that really.
As long as the PCB does not reach 100 degrees Celsius for a prolonged time you will be OK. Around that temperature the solder might start to deform/melt and cause the BCM chip to get dislodged or make cold solder joints and cause problems.
You really do not want to expose it to anything above 50 degrees Celsius at all.
The inside of a car can get really hot anyway so best place is to hide the Pi under the dashboard somewhere. Like near the foot section there usually is easy access. That will be fine. Radios are also 99% hidden behind the dash and ECM units as its cooler there. So that is not a problem for any electronics really. But keeping it in a box does help even when you hide it under the dash.