I hope I can give you a general Idea. The radiator of the vehicle having different configuration setup whether you are considering a coolant-oil exchanger or not.However, in simple setup you can consider the heat transfer coolant side to the external air by considering the system as a series of thermal resistances for a 1D simple-steady heat flow analysis

convection->conduction->convection

considering the radiator area ,A  and length, L 

heat flow,q=Q/A (q , Q are flows)

• Coolant side : (  Temperature of coolant - Temperature of coolant,wall) = (1/h,coolant)*(heat flow)
• Wall side: (  Temperature of coolant,wall - Temperature of air,wall) = (L/k)*(heat flow)
• Air Side: (  Temperature of air,wall - Temperature of air) = (1/h,air)*(heat flow)
• The average values of the conduction and convection can be taken as h,c=3000 W/(m2K) , k= 200-400 W/mK ( depending on the radiator material) , h,a= 100 W/(m2K) ( This gives the main contribution-> air side)
• Kr= 1/(1/hc + L/k + 1/ha)

Then by considering the final result we have Q=Kr*A*(T,coolant - T,air)

This is a very simple model , however the radiator is much more complex than a metal plate as considered above because the global heat transfer coefficient also depends on the type of technology you are considering ( braze-welded or mechanical welded) or may be a full aluminum radiator . You can also consider the ATB ( Air to boil ) index or ETD ( External Temperature difference ) to know the safety limit for the coolant temperature. 

This is a good question with lots of possible answers.  In addition to the very helpful answer given by @Usman Salim, there is a bit more to add.

For an introduction to the mathematics of a polymeric hollow fiber heat exchanger as car radiator, see Section 1 in

https://www.researchgate.net/publication/305728841_Polymeric_Hollow_Fiber_Heat_Exchanger_as_an_Automotive_Radiator

A mathematical model used to study the effect of radiator fins blockage by clay soil is given in Section 2.1 in

https://www.researchgate.net/publication/279841751_Effect_of_Radiator_Fins_Blockage_by_Clay_Soil_on_the_Engine_Cooling_Temperature

Article Effect of Radiator Fins Blockage by Clay Soil on the Engine ...

Article Polymeric Hollow Fiber Heat Exchanger as an Automotive Radiator:

 http://www.iosrjournals.org/iosr-jmce/papers/RDME-Volume2/RDME-11.pdf