i have an old furnasman gas furnace.

It has AC

there is a honeywell transformer screwed on to the electrical connection
enclosure i suspect it was installed when the ac was installed.

I want to replace the fan motor with a 2 speed one

i want to add a switch to the furnace so i can have the fan on all the
time in slow speed. then when the burner or the ac comes on it will
change to high speed.

is there a relay i can install to do this or is there something else i
need to do.