98 jimmy low beam not working

I have a 98 gmc Jimmy that had electrical work done to pull a trailer since then when the brakes were touched the low beam headlights went out and when you took your foot off the brake they went back on. Get home and unhook trailer check to see loose wires etc... not! Now the automatic lights that gone on when it is dark (low beam) doesn't work (high beam does)! Checked everything and I mean everthing what am I doing wrong?? Hate to bring to dealer and find out this was very simple to fix myself!!!