Radio deck

My 2002 mustang has the stock deck to play
music. I use a cassette adapted to play music .
Recently I took it to the mechanic and when I get it
back , my radio and my speaker weren't as loud. I
bought a new adapter. But I noticed when I put in
the tape , a letter C pops up . The only letter C I
know on the radio is a button named comp. when I
put a cd in I could push that button and the same
letter C will pop up. But it lets me turn it off in cd
mode. I'm assuming is that same letter C but it
won't turn off in tape mode .... Hope this makes
sense