this was done a lot with Dynaco's PAS2/3. on the circuit diagram follow the original single phono signal
from the input RCAs, to and through the selector switch to its "final" destination at the phono stage
(usually a tube grid but could be on a circuit board - I'm not familiar with fishers). mark that location
with masking tape or a marker.
then trace the tape head input to the selector switch (this assumes there's NO parts in the signal path).
From there note any differences (caps and resistors for whatever the tape head equalization was designed for)
from the same path the phono signal took. then note where the signal ends - probably same general
location as the phono.
now wire the tape input the same (same parts or no parts and removing all tape head parts) and to the
same end destination.
the switch will select the designated phono input, they both end up at the phono amp section where
the phono RIAA filters kick in.
label the "tape" to phono or TT2.
this is a lot easier if a schematic is posted, then you could wire the tape signal path directly into
the phono sections. probably two wires at best case.
I've made some assumptions having never looked at a Fisher but you should be able
to follow my logic which should be the same as your intent.
good luck
