Still sounds like an earth problem to me.
Are you sure the loose cable was an earth? If so, that you soldered it to real earth?

Only thing you can do in these cases is bring in some logical checking.
Check the main POS and NEG cables on the battery (no corrosion)
Check if the black earth cables connection on the engine or frame is clean and not corroded

Then move over to the effected area, and slowly work your way through all connectors.
Disconnect and connect them one-by-one (incl. the bulb fitting of each lighting bulb), then in small groups and see the results on all lights.
If you do this systemetacally, you will most likely find the culprit.
After that, you can check it is the bulb, the fitting, the conectors or a cable problem.

E.