The datasette is accessed via $01 (right, it's directly connected to CPU)
You have 3 lines = 3 bits in $01:
bit 5 is Cassette Motor Control (NOTE! It's low-active: 0 = on; 1 = off)
bit 4 is Cassette Switch Sense: 1 = Switch Closed
bit 3 is Cassette Data Output Line
Make sure you don't mess up $00!
Since bit 4 is input and bit 3 obviously output.
Default for $00 on C64 is %00101111 ($2f)
And in case you wondered: you READ from tape via the FLAG-line in CIA1:
bit 4 in $dc0d.
In BASIC $c0 (dec.192) is a tape motor control too…