I think you might need separate scripts.
#loop
?(player in car1) : goto "add"
~0.01
goto "loop"
#add
player addAction ["Turn on the car radio", "radio.sqs"]
exit
#loop
?!(player in car1) : goto "rem"
~0.01
goto "loop"
#rem
player removeAction 0
this exec "Musiccar3.sqs"
exit
0.1 fadeMusic 0
player removeAction 0
this exec "Musiccar1.sqs"
exit
_random = random 11
_a = _random - (_random mod 1)
player removeAction 0
player addAction ["Turn off the car radio", "Musiccar3.sqs"]
~(random 3)
?(_a == 1) : goto "1"
?(_a == 2) : goto "2"
?(_a == 3) : goto "3"
?(_a == 4) : goto "4"
?(_a == 5) : goto "5"
?(_a == 6) : goto "6"
?(_a == 7) : goto "7"
?(_a == 8) : goto "8"
?(_a == 9) : goto "9"
?(_a == 10) : goto "10"
?(_a == 11) : goto "11"
#1
playMusic "res1a"
exit
#2
playMusic "res1b"
exit
#3
playMusic "res2"
exit
#4
playMusic "res3"
exit
#5
playMusic "res4"
exit
#6
playMusic "res5"
exit
#7
playMusic "res6"
exit
#8
playMusic "res7"
exit
#9
playMusic "res8"
exit
#10
playMusic "res9"
exit
#11
playMusic "res10"
exit
Note that "radio.sqs" will play only Resistance soundtracks. The script is easily modifiable, so modify it to suit your needs.
Edit: Found few bugs, fixed the scripts.
Hope that helps,
Krieg