It's called a back feed, and it happens when there is an insufficient
ground, and the electrical loads back feed thru other sources that
have better grounds, this can cause all sorts of really strange things
to happen, like wipers coming on when you hit the turn signal, and
it's just a bad ground connection.
At first you think there is a problem in the multi stalk on the steering
column, but not so
My advise is don't tear into your dash wiring first, check the body ground
from the battery, and then the rest of them, that means take it loose,
wire brush all points of contact clean bare metal and put back together.
Also make sure the small wire from the battery ground to the body is really
making a connection, and that it isn't corroded to copper powder inside the
insulation cover
Wild