eveything in my car works,radio,signals,all accessories but when i turn my light switch on it kills my car dead and i dont get any power to my fuse panel.sometimes my car will start right away sometimes not for hours.i have traced power from battery to firewall eveything reads fine no loose or corroded connections,it just seems that my switch is shorting out ,is it possible that the power goes to the switch before it sends power to the fuse panel? i dont know how to test a switch with my meter,would a gm code reader tell me whats wrong?do they tell you whats wrong or just give you a number ,it seems like it comes with a thick little instruction manual.

 

I guess I'll help you out. Sounds like you have a bad ground. If you have a bad ground, then turning on a high current accessory will increase the resistance at the bad ground. This would make stuff like the engine computer shut off, or your radio, or anything else.

Is the car rusty? Check the battery post connection, where it screws to the engine bay, under the dash at the ECM, under the dash at the kickpanel and by the steering column. Those grounds are important! Other than that, more info is required. Good luck.