On the core support, drivers side of the radiator.
The 1994 Pontiac Firebird horn relay switch can be found on the inside of the front quarter panel. The horn relay switch will be on the drivers side quarter panel.
Under the dash above the break pedal. Look above your horn relay and it is a round relay, that is your flasher.
Pull out the old flasher (located under dash pannel above brake pedal, above the horn relay) and put in the new flasher.
Try the driver side core support on the engine side. That's where my 68 is located. Sorry I couldn't be more specific for your year of 'Bird.
See stvndberryhill's answer under 1998 Pontiac Sunfire horn relays. This answer applies to all year models of Pontiac Sunfires.
Did you try the horn relay? It is located near the fuse box.
where
where is the horn relay located for 1978 lincoln mark v
Horn relay Horn fuse steering wheel switch
Horn relay Switches at steering wheel
On the firewall to the left of the wiper motor
Have your Pontiac dealers parts department print out a picture of the parts location for you. They will do this if you ask them.