I had the same problem awhile back , trace the wires going to the wiper motor itself , you'll find it has a plug in harness , on the harness that plugs into the motor you'll find a thick ground wire , cut that wire , peel it , attache another wire the same gage , use a wire o-ring connector , attache the other end of the wire with the o-ring on it to the negative battery post ,I think it's either a 14 or 16 gage wire as I recall.
Possible others causes are rotted or corroded wiring going to the wiper motor or the multi function switch on the steering wheel is faulty or the body control module=BCM is faulty or again wiring going to them.
Hope that helps and best of luck.By the way another thing you can do if you've no wire handy is trace the black wire I mentioned to it's ground location " I think it goes to the frame by the battery" remove the screw , clean that wires connections and any other ground wires you find.I'd highly recommend that you adventually change all those ground wires and make up a whole new ground.
