First thing you should do is have it scanned for any control module problem, then testing is as below:
One thing to call your attention to is this note:
This step and this flow chart are a match to GM factory information. But in step #5 of this "Wiper Inop - One or More Modes" flow chart, the test has you check resistance in Mist, LO and HI positions. According to wiring, HI speed wiring is bypassing the 390ohm resistor! Therefore. you should not get 390 ohms on HI speed. If you check this same test under a 2005 Silverado/Sierra, there is no reference to checking HI speed resistance on step #5. Use your best judgement on this step, try answering "yes" if you get the correct 390ohm spec. on Mist and LO (as per Silverado/Sierra testing), but you make the final call on this possible GM error.