On my 75 Coupe behind the passenger side dash panel there is a heat sink with a transistor attached on it.  I'm just curious what this is for.  On mine it is disconnected so I removed it.

It most likely was part of the original radio.  There was one in my '70 so would assume that the audio electronics hadn't advanced much by the time your car was built.

What he said... That's the "amp" part of the factory radio. The space available for a radio in these cars was tight, so they had to separate the amp from the head unit.