Your radio reception changes are due to either an electrical issue or a mechanical one. Here are some things you can test.

Dear Car Talk

Dear Car Talk | Sep 01, 1996

Dear Tom and Ray:

I think I may have you both stumped, but I hope not. I have a 1991 Ford
Taurus. When driving with the radio on in the AM mode, I get nothing but
static. However, as soon as I apply the brakes, it's as clear as a bell. I
release the brake pedal, and it's back to static. How come? My Ford garage
wants $35 an hour just to look for the cause, and they say it could take
several hours. How about putting your heads together and giving them a head
start (so I can save some big bucks)? -- Joe

TOM: Problems like this are invariably simple to fix, Joe. It's finding
them that'll drive you nuts.

RAY: There are two basic possibilities, Joe. It's either a mechanical
problem or it's an electrical problem.

TOM: A mechanical problem means that something is being physically moved.
Here's how you find it. First, turn the radio on and get under the
dashboard. Try applying the brakes with your hand, and see if there are any
wires being pushed around by the brake pedal. It's possible that the
antenna wire (even though the antenna is on the passenger side in this car)
or ground wire is getting moved momentarily by the movement of the brake
pedal, and improving the reception.

RAY: Once you see which wires are moved by the pedal, try reaching up there
and gently moving the wires by hand (without pushing on the brake). If you
can get your AM reception back that way, then you've eliminated the brakes
from the equation.

TOM: If you can't get your reception back by moving the wires, then you've
got to pursue the "electrical" angle. The only two things operated off the
brake pedal are the brake-light switch and the cruise-control defeat
switch. And I suppose it's possible that one of those is somehow completing
the antenna or, more likely, the ground connection. How? As your dealer put
it, Joe, I would have to charge you $35 an hour for that explanation, and
it would take several hours.

TOM: So the next thing you do is disconnect those two switches one at a
time. Start by unplugging the brake-light switch. If stepping on the brake
won't get your AM reception back now, you know the brake light wire is
involved. Then you just have to learn to ride the brakes while listening to
the ball game.

RAY: Actually, if you take it that far, and narrow it down to the brake
light switch or cruise-control defeat switch, you'll be giving your
mechanic a huge head start. And instead of charging you three hours labor
to diagnose the problem, he'll probably just charge you two hours labor for
confirming your diagnosis. Good luck, Joe.

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