In article ,
Meat Plow wrote:

On Fri, 28 Sep 2007 20:13:10 -0400, Bill Freeman wrote:

I have an 87 Nissan van with an antenna wire that has a 0.85 mfd
capacitor soldered between the center wire and the center pin of the
antenna connector, on the end that plugs into the radio.

This vehicle actually has two antenna wires that connect side by side
at the radio. One antenna is printed onto the centerline of the
windshield, and the other is a conventional whip antenna mounted at the
drivers side of the roof. The roof antenna lead is the one with the
capacitor.

Anyone know what the purpose of an inline capacitor would be on a car
radio?

Reduces engine noise on AM.

Another car antenna question, why do they use such a small diameter
wire inside the coax? The wire inside this coax is about .010 inches
in diameter. Is it this small to save on copper, or is there some
technical advantage to a small diameter wire.

Some technical advantage, maybe tuning the antenna circuit for AM.

Although I think Arfa recently met an audiophile who could supply the OP
with an $1800 coax, whose center conductor is made of 300 strands of
52AWG gold wire, specially braided to optimize the FM band.