...sounds like its time to do a complete cooling system check-up which will include but not be limited to (its imperative for a 500e cooling system to be at 100% effectiveness):
>fresh OEM MB coolant in the correct % to distilled H20 (check service records, needs to be done every 2 years), correct coolant level, no coolant leaks
>review correct cut- in of low, med, and high speed Aux Fans (resistor/ fuses may need replacement)
>review correct function of clutch fan
also helps to review cleanliness of the air filter(s)- you have 2 of them....if they're dirty and clogged- - it won't help matters starving the motor for air
>in a healthy 500e, the low speed fan will be on w/ a/c on...
do a search to find the info on med and high fan temp cut in's
>>>JimF's cooling piggy tail is great "insurance" and appears to be an effective "add-on modification" when mated to a healthy and 100% effective 500e cooling system (not a great answer when mated to an ailing system)....<<<mated to an ineffective system, you'll probably just burn up your fans prematurely as they'll be on 100% of the time
good luck and let is know how it turns out!