Don't know if this is the problem in your case but I would suggest
carefully checking out the fixings around your rear lights - this is
often a sign that the earth connection has gone faulty or Hi-Res!

HTH

Regards,
John

Years ago I recall a common problem - exclusively with Falcons as I
recall - whenever they changed gears the backing light lit momentarily.
Pathetic.

I have had this problem twice on different cars over the years.   On each
occasion it was caused by a faulty stop/tail double filament bulb.   One
filament had broken and attached itself to the other, thus feeding power to
the side lights when the brake lights were operated.

Yes, I'll give you £300 for it! (Providing it has less than 70,000 miles on
it)

Artie