What is this?

Hey guys
1978 GTS US
The drive cable broke in the power antenna. Turned out to be an easy fix once I figured out how it worked, however, while I had was working on the antenna I noticed this plastic electronic component enclosure spliced in into Image Unavailable, Please Login the antenna cable. I, of course, have seen it before but never thought about it. Just assumed it was a "signal booster". As far as I know It is original to the car (maybe dealer installed), It was there in 1980 when I bought the car. Its about 2" sq sealed plastic box with no part number. It has no power going to it and my ohm meter shows no through put for either the sheathing or primary. Radio reception seems about the same with or without it, however, the radio does have very poor reception if the antenna is down with or without it which leads me to believe the thing is doing something.
Anyway no big thing. Just wondering. I'm leaving is out of the car for now.

Thanks
Barry

 

Its antenna connections on two ends, but not powered, so must be passive inductor device?

 

Not original.

 

Might be a passive bandpass filter. Only lets signals in the frequency range of the AM and FM bands, but blocks extraneous engine signals like from the ignition, alternator, etc.