orchidskip,

I think a simple method would be to tag a wire off the stoplight switch to the light bar. This would bypass the turn signal conglomeration and only allow the light bar to illuminate in a steady fashion when the brakes are applied.

I think there is also a little solid state device you can buy that attaches to a brake/turn wire and negates the blinking, giving you a steady light. This is how modern rigs with the third light work.

Stuart