To elaborate on what Kurt said (that the 'Bird came with a rear FM antenna), it wasn't a "fixed mast" antenna like the Camaro FM antenna! Both the AM and the AM/FM radios in the 67-68 Firebird used the same three section manual telescoping antenna! "Normal" mounting position was on the front fender (like the Camaro), while optionally (RPO 381) you could have it installed on the rear fender.
Late in the 1969 model year, the factory discontinued the manual rear telescoping antenna and went to a three section POWER antenna, also RPO 381. (Here's a picture of the
WINDOW STICKER and a picture of the
SHIPPER showing the option, and a picture of the
CAR itself.)
HERE'S a great link to the 1968 Firebird Sales manual, with the specific antenna option page
HERE.
Also, a GREAT First Generation Firebird
FAQ PAGE with specific info on the antennas
HERE.
I've also attached a picture (below) of a 67 'Bird with the rear manual rear antenna.
The three section telescoping antenna may have been the key to "tuning" the antenna so noise could be kept to a minimum, so I agree with Kurt. Just hook it up and see what you get.
Ed