Most likely the resistor. They fail all of the time.
The radiator fans in the Boxster run at two speeds. The low speed runs when the car engine temp is higher than 206°F or the A/C is on. The high speed runs when the coolant temperature is higher than 215°F or the A/C freon pressure is higher than 16 bar. High speed fan operation simply goes straight to the fan. The low speed is created by sending power through a ballast resistor which reduces the power to the fan and creates the low speed operation. The resistor is prone to failure.
Here is the test sequence that I followed:
(1) Fuses
(2) Relays
(3) Fan motors
First, check the radiator fan fuses, Row C, #8 and #10, 40A.
Next, test the relays - Start a cold engine, trans in neutral with parking brake on. Get down on your knees and put your left hand up on the low speed relay (either #19 (left) or #21 (right)), then reach up and turn on the A/C with your right hand. If the relay is good, you'll feel a "click" in the relay and the fan will turn on in low speed. Turn a/c off and the relay will click again. You can also do this test using a Durametric to turn the fans on to low (my version of Durametric does not have high/low, just on/off and on is low speed). If a relay fails to click, replace it.
Next, test the fan motors by jumping the relay to force the fan to operate at high speed. Remove both the low-speed (#19 for left) and high-speed (#20 for left) relays for the fan that isn't working (or #21 and #22 for right). Cut and strip a piece of wire about 1.5" long. Insert one bare end in the output socket (pin 87, center bottom horizontal slot) for the low speed relay and the other end in the output socket for the high speed relay.
With the jumper in place, put the relays back in and test. Start the car cold, turn on the A/C, and verify that the fan is now operating (it will be at high speed because of the jumper). If the fan is running at high speed with the jumper, then the ballast resistor (which controls low speed operation) has failed. If not, then the fan motor itself has failed (because it won't operate at either low or high speed).
A replacement resistor is about $100-$125.