This question is about the 69 Shelby but pretty much applies to mustangs. The horns work just fine, tested. good ground to the core support. . The rimblow has a new repo switch in it. When squeezed the interior lights dim and the horns make a very weak attempt at making it's sound. I hooked up a horn push button to test it and the same effect. So I believe the rimblow part is working. It seems a grounding problem and could this mean a problem with the tilt signal switch inside? Any thoughts or idea's would be appreciated.