2001 Chevy Express brake lights

When just the daytime running lights are on all lights around the van work properly. When I turn on the headlights and step on the brakes the back brake kights go out but the third brake light works properly. My mechanis has replaced the headlight switch assembly abd the brake light switch to no avail. He now tells me that the multi function switch in the steering column must be the problem. He alrady has 9 hours of labor in the van and this new switch costs over 300 dollars. I was able to return the first wo switches but the dealer tells us that once this switch is in it cannotbe returned. Do you have any ideas on this problem.