How do I get all my new rear lexus lights working on my peugeot 206?

Fitted new lexus lights to my peugeot 206. Everything was fine all lights working etc. But when I have my head lights on at night my brakes lights don't work. Not very good eh? So I thought I can't have no brake lights, put my old lights back on and now they are f**ked. When my headlights are on my 3rd brake light on back window stays on all the time now. Not as bad as having no brake lights but not so good either. I want to know how to fix my brake lights on my lexus lights as they are the ones I want on my car but if you know how to fix my old ones that would help too for now. Don't want to take it to an auto electrician and cost me a fortune. Please help me!