What happens when you put the turn signal on? Windows go down?
Sorry, kinda funny situation you have. In a lot of cases it can be a poor ground for one or many devices. Electricity will try to ground it self to complete the circuit, sometimes affecting other devices (lights go on/off, horn sounds). It would be a good idea to make sure everything is grounded properly on your car and then do further troubleshooting.

Someone on this site may have a diagram for all the grounding locations. Make sure your ground straps are clean and installed.
They exist in your engine compartment wiring harness, engine to frame, interior (cab and trunk) wiring harnesses and the instrument panel.

Hope that helps ... beep beep