Check engine with trailer connected

[Larry Newman]

I recently installed a trailer hitch and wired my '89 200 for a 6 wire
trailer umbilical.  Whenever I connect the plug to the trailer, my Check
Engine light comes on and indicates a brake malfunction.  I've been told
by an Audi owner who tows a race car that this always happens and there
is nothing one do about it.  Anybody familiar with this phenomenom and
have discovered a way to have the trailer connected without lighting up
the brake problem symbol?

Larry