As to the bulb failure light problem i have kind of same thing after mounting a trailor hitch. Having connected the trailor brake light to the original wiring in the rear left "hole" in my 745 the bulb failure light won't go off after 1.st braking as usual - it dosn't go out 'till i disconnect the the trailor light plug.
Talking to a Volvo garage recently i was given this advice as to correct wiring: run a seperate wire for brake light from the trailor plug all the way to the brake light contact at the brake pedal !
Haven't had time to test it yet (only just started towing my boat with the warm weather finally here !).
But somehow it sounds right; the brake light does seem to "test" the brake failure lamp and turn it off first time you apply the brakes, so maybee adding an extra set of brake lights somehow "overloads" the system!

Jesper