[Johneve]

Personally, I don't see the point in re-pinning the connector, the problem will just come back.

If you solder a length of wire to both ground planes inside the the lamp unit and then lead it out through the vent hole on the back of the unit (so no holes to drill) taking it on down to the earth stud in the boot you have a sure-fire repair for the total cost of a length of wire.

 

[Johneve]

That's the problem with nut and bolt method, you may not get a good, permanent connection. I admit soldering to the ground plane wires takes a 100W or greater soldering iron and a little patience but you do get a certain connection that will last for ever.

 

[Johneve]

The ground planes inside the light unit are attached to the plastic case periodically with plastic stubs melted over the metal. You need to choose a spot to solder between the plastic stubs so you can get the metal hot enough without melting the stubs. I used a Weller instant heat 100w soldering gun.

You're right these lamps are just too sensitive. I started by fixing my LHS unit because that was the one that was giving me errors and the earth pin looked a little discoloured but not burnt. Having fixed that one, I tried the lights and it was obvious that the RHS unit was darker than the newly fixed LHS one. The RHS one wasn't giving any errors and its earth pin in the connector looked perfect. I think it was a marginal design from the get go. I then did the same repair to the RHS and had equally bright rear lights and no more errors!

It's also an opportunity to replace all your bulbs with new Osram or similar quality bulbs since even if still working the original factory bulbs will have darkened significantly.