From what I've read on this, the stock fan pulls 15amps ish and can peak at 30amps on low speed, and 25amps ish with a 75amp peak. I've put one in my base T-Bird and had to use 2 40 amp relays in parallel to run it (high speed on a switch, low speed on a controller).
The stock setup uses the IRCM and EEC to turn the fan off and on. The IRCM is essentially 3 relays (low speed, high speed and a power relay that supplys +12v to the coils on the other 2). The 2 relays that actually power the fans use a 16 ga fusible link.
You would probabaly want to hook both of the stock hot wires to the hot side of the aftermarket fan. Using just the low speed would work most of the time, but if it were to get a little hot the EEC will shut off the low speed and turn on the high speed. Using the high speed alone would make it turn on a little late for my tastes. I have the wiring diagram for the stock setup if you need it.