I get static in the AM band on the radio when the station is weaker than the strong local stations. I can also hear the spark plugs sometimes and get a squealing sound that warbles sometimes.

It varies if I touch the brake pedal turning on the brake lights and goes away for a part of a minute if I turn on the rear defroster and turn the defroster off. I suspect a ground connection or a connector for the antenna in the rear window since it varies with the rear defroster being turned on.

Where do I start? 138K LeSabre

Friends don't let friends AM.

Ok, change your plug wires for starters. Other than that, there'* not alot you can do, other than an antenna signal booster, and I think the AM is internal only in the HU, so if that'* true, it would rule that possibility out.

I know the boosters work well for FM.

Isn't this why FM was created? Because AM while getting reception over long distances .... isn't very high quality.

The radio picks up the station fine--after the rear window defroster pulls a current for a second. Then it last for a minute or two and the static gradually returns.

Before this the station may have been great for 10-15 minutes after the car was started and driven.

The problem is NOT signal strength too low--they are stations that come in fine. IT'* just that they're not the 50000 watt WLW type station. I'm only 50 miles from the station.

AM is horrendous for picking up interferance, in a car or not.
My car does the same thing, I can hear the RPM'* thru the radio on weak AM.
I even have an aftermarket stereo- Pioneer.

Of course I do not listen to AM

I will tell you just how bad AM is - They say when trying to detect electrostatic interferance, use a cheap AM radio, put it on a non-station low frequency, and hold it close to items you suspect of putting off a lot of ESI.
This is the procedure written in a computer repair book I own about when one gets weird computer problems.

Except for MAYBE using a filter in the power line to the radio, there is not much you can do.

FUN QUESTION - With XM, FM, MP3, CD,... etc, how is it that AM is still even alive? :?

Noone has hit the point yet.

The radio works okay after the rear defroster is turned on, or the brake pedal turning on brake lights sometimes helps. There is something changing in the car'* electrical system. It is not the weak AM station.

I'm suspicious of the alternator and regulator in it. If the brushes are worn down at 130K miles, would they be more susceptible to not producing a solid output current. But when the load increases they make good contact again and the electric current becomes solid again?

It'* a possibility, but it may only be helping an otherwise weak station.

If you're serious about your AM listentening, get a new alternator (don't go cheap, and get one with a warranty), and replace your battery at the same time if it'* over 2 years old.

In addition, GOOD quality plug wires.

Check your grounding. There is a copper ground strap at the antenna, bonding the antenna shield to the car'* body. Check the connections at either end of the ground strap, and remove any excess paint or corrosion here. Also try checking the antenna connections, both at the antenna and at the radio. AM radio needs a good ground plane (the body of the car) connected to the radio to get the best reception.