1985 w126 300sd headlamp won't turn off

[lotsmorebetter]

This is very frustrating and not the first time it has happened.
Day or night, the headlamps and signal lights will come on by themselves and won't turn off without unplugging them at the connection harness.
I have replaced the headlamp switch twice in the past with used switches and then most recently with a new one, and while that worked for about 6 months, now it too doesn't help.
I have looked around at other sites to see if there is either a relay involved or ANY other solution, but I have found nothing to help.
HELP!