The electric fan(s) should be on pretty much any time the A/C is running. Ambient temps over 85 degrees should absolutely have the fan on if the A/C charge is correct and the compressor is running. This will be low speed, not high speed and is designed to help the A/C work better, not to cool the radiator.

The fan clutch is thermostatic. If air is not reaching the thermostatic element, it will not engage. Make sure that the radiator and A/C condenser are clean, they can plug up with debris and block the airflow to the thermostatic element. If the engine is running hot and the cooling system is working as it should be, that fan should be engaged and moving a large amount of air. If you open the hood and rev the engine, you should hear it whining and see/feel a substantial increase in airflow from the fan.

New doesn't always mean good. Behr clutches can be hit or miss. Verify that it's working properly before dismissing it as a possibility.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]