recently, when i use the rear defrost it knocks out my radio. any ideas.
It’s usually broken traces in the defrost grid. There are repair kits available for some cars.
You have a short someplace.
Why do you say that? I am assuming that the OP has one of the many cars that use the defroster grid for the radio antenna. Everything I’ve read, indicates an open in the grid, not a short, is the problem.
An open grid will stop the defogger from working…NOT knock out the radio.
Not a completely open grid, just some of the lines going across. But, by what magic would the radio be able to keep working if its antenna (the defrost grid) were gone?
I wonder if the defogger and the radio antenna share a ground that might not be making good contact.
Does the radio go out as soon as you turn on the defogger and does the radio come back at the exact same time as you turn the defogger off?
What kind of car?
Consider adding a dedicated antenna, they always give better reception espically on AM.