I have a bad earth on my Ford KA Tailgate

Sounds like there might be an electrical short in the system. You could try replacing the wiper motor. It's not a difficult job to do yourself just a bit time consuming and fiddly.
Unclip the cover on the wiper arm and unscrew the nut until it is only on 2 or 3 turns. (This stops it smacking you in the face when it suddenly lets go!). The arm should pull towards you. You might need a puller tool if it's stubborn.
DON'T use a hammer as you could shatter the rear window glass! There may be a rubber plug on the shaft sticking out. Don't lose this as it stops water getting into the tailgate.
Unscrew and remove the inside trim panel on the tailgate. This should reveal the motor and the 3 bolts holding it in place. Disconnect the electrics first THEN unbolt the motor. It should just lift out.
Refitting is this method done backwards.

Read through these instructions. They go over the front then the rear motor replacement.

http://www.ehow.com/how_12206366_replace-windshield-wiper-motor-2007-honda-crv.html#page=15

There is a nut under the black plastic bezel that needs to be removed. Here is the complete procedure for you to look at. Its possible that your arm isn't adjust at wiper motor...

REMOVAL
- Disconnect and isolate the battery negative cable.
- Remove the rear wiper arm from the rear wiper motor output shaft. (Refer to 8 - ELECTRICAL / REAR WIPERS / WASHERS/REAR WIPER ARM - REMOVAL).
- Using a small thin-bladed tool, gently pry at the notch in the base of the rear wiper motor output shaft bezel to unsnap the nut cover from the bezel (REAR WIPER MOTOR OUTPUT SHAFT NUT REMOVE/INSTALL). Be certain to take proper precautions to protect the outer tailgate panel and its paint finish from damage during this procedure.
- Remove the nut that secures the rear wiper motor output shaft to the outer swing gate panel (REAR WIPER MOTOR OUTPUT SHAFT NUT REMOVE/INSTALL).
- Remove the bezel and gasket from the rear wiper motor output shaft.
- Remove the trim panel from the tailgate inner panel. (Refer to 23 - BODY/SWING GATE/TRIM PANEL - REMOVAL).
- Disconnect the tailgate wire harness connector for the flip-up glass ajar switch from the flip-up glass latch connector receptacle.
- Disconnect the tailgate wire harness connector for the rear wiper motor from the motor connector receptacle (REAR WIPER MOTOR REMOVE/INSTALL).
- Loosen the two screws that secure the rear wiper motor mounting bracket to the top of the tailgate inner panel.
- Slide the rear wiper motor and mounting bracket forward far enough to disengage the two mounting screws from the keyed slots in the top of the tailgate inner panel.
- Remove the rear wiper motor and mounting bracket from the tailgate as a unit.



1 - BEZEL AND GASKET
2 - OUTPUT SHAFT
3 - TAILGATE OUTER PANEL
4 - NUT
5 - NUT COVER

What you will need to do is to remove the small plastic cap on the wiper arm by prying it off with a screw driver. Then remove the nut that holds the arm onto the shaft. Then get yourself a battery cable puller (from auto parts store) and pull the arm off of the shaft..



1 - SPARE TIRE
2 - WRENCH
3 - FLIP-UP GLASS
4 - PARK RAMP
5 - REAR WIPER ARM
6 - PIVOT COVER

yes the wiper motor is probably at fault due to the self parking mechanism is internally in the motor.
well the rear one check fuses and then check for power at motor, if there is power at motor possibly motor again, if there is no power at motor when operating check wiring in flexible joint between body of car and back door/tailgate.

REMOVAL

1. Disconnect and isolate the battery negative cable.
2. Remove the rear wiper arm from the rear wiper motor output shaft.
3. Using a small thin-bladed tool, gently pry at the notch in the base of the rear wiper motor output shaft bezel to unsnap the nut cover from the bezel .Be certain to take proper precautions to protect the outer tailgate panel and its paint finish from damage during this procedure.
4. Remove the nut that secures the rear wiper motor output shaft to the outer swing gate panel
5. Remove the bezel and gasket from the rear wiper motor output shaft.
6. Remove the trim panel from the tailgate inner panel.
7. Disconnect the tailgate wire harness connector for the flip-up glass ajar switch from the flip-up glass latch connector receptacle.
8. Disconnect the tailgate wire harness connector for the rear wiper motor from the motor connector receptacle
9. Loosen the two screws that secure the rear wiper motor mounting bracket to the top of the tailgate inner panel.
10. Slide the rear wiper motor and mounting bracket forward far enough to disengage the two mounting screws from the keyed slots in the top of the tailgate inner panel.
11. Remove the rear wiper motor and mounting bracket from the tailgate as a unit.

To Install just reverse steps

Sounds electrical. Check fuses and connections.

You have already solved your problem. Connect the two ground points with good flexible wire and tie it up with plastic ties so that it is out of the way and near a hinge so that there is minimal movement. The original scheme probably relied on conduction through the hinges for grounding.

Check the wires that come through the tailgate, constant flexing may cause a brakw in one of the wires also with all these stopping at the same time they probably share the same ground. If you apply a ground wire to the circuit and it works you must find where it is broken and repair it.

the switch needs to be replaced or theres a bad relay involved.

Remove the outer wiper arm. Remove inner tailgate coverings. Disconnect wiper electrical connector and retaining bolts to remove. Installation is reverse of removal