DIY Drive-In Radio Sound System

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I recently became a fan of the old drive-in movie theater experience. I like parking the car up on the berm, relaxing in the seat, eating movie snacks, yacking within the sound-proof confines of the car and enjoying the movie. However, I became concerned about the car's battery drain from operating the car's sound system (and air conditioning system) after listening through two movie soundtracks. My fear is that after the movies are over, my car's battery cranking power will not be strong enough to start the car.

I want to make a stand-alone "Drive-In Radio Sound System", consisting of an ordinary car radio (not connected to the car), powered by an old motorcycle battery (it carries a 12V level, but is no longer strong enough to start a motorcycle), connected to a portable boom box (powered by 12 size D Lithium batteries).

The power to car radio (from the motorcycle battery) needs to last four to five hours, with enough juice to receive a clear, strong FM audio (stereo) signal from the drive-in theater broadcast channel.

The wiring will also need to be modified to adapt the car radio's single speaker wires (i.e., YEL+RED to the positive battery terminal, BLK to the negative battery terminal, PURPLE and PURPLE-BLACK adapted somehow) to the audio inputs of the boom box. The boom box does not have a standard positive or negative press-down connecting slots like ordinary speakers, but there is a USB input slot as well as red and white RCA female jacks.

I brought a TDK Sound Cube to the drive-in and tried to pick up the FM channel that broadcasted the movie soundtrack, but the Sound Cube failed miserably. My boom box, placed outside the car, with the attached antenna fully extended and oriented back (straight line-of-sight) towards the broadcasting tower, was not able to receive a strong, clear FM signal. Instead, I received a very weak signal that cut out frequently. However, when I operated my car radio system, it worked like a charm. I was able to receive a strong, clear FM stereo signal. I researched this behavior online and (correct my knowledge) discovered car radios have a better antenna than boom boxes.

With the items I have below, can I make a DIRSOSY that can achieve the following requirements:
1) receive a strong, clear FM signal
2) operate continuously through two whole movies
Or...
What additional items should I buy that would meet the above requirements and make this system work?

-An old car radio (Scion Pioneer, T1807, SAT Radio Ready)
-An old 12V motorcycle battery
-A TDK Life on Record Sound Cube
-USB input
-3.5mm audio jack (female, white and red) inputs
-AC power supply input
-12 size D battery power supply input

:)
DIRSOSY
"Total Noob to Radio"
 
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