instrument lights

Ok I have a small problem when my headlights are on my instrument lights go off but with my four ways on the instrument lights are on. I replaced the switch and it still happens any ideas

this happened on my friends old bmw that we were messing with, i found it to be a problem with the wiring in the fuse box. that car was so screwed up lol we ended up getting it running sticking the throttle and having it drive into a cliff in the mountains, of course we cleaned up our mess and threw the thing in the junkyard but it was fun lol