Been there, done that.
My 190e had the same problem. On mine the fans are connected in parallel, they should both be doing the same thing at the same time. Each had its own connector clipped behind and near the bottom of each headlight. Just unplug them and put 12v or so to them. Don't worry about polarity, if you get it backwards the fan will just spin backwards. I would suggest using a battery charger, its safer than using a 1000 A (when shorted) battery.
I fixed my bad fan, just took it apart and found one of the brushes got stuck.
5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)