what dictated, in 1968, whether the antenna was on the front fender or the rear fender? Was it an option as part of any of the packages SS or otherwise? Or did it have to do with what type of radio came in the car?

First off - WELCOME to Team Camaro graemlins/beers.gif

The standard Antenna was front mounted with any radio option.
The 'Rear Antenna' was an option (RPO-U73) only with the optional AM Radio (RPO-U63), not with any of the AM/FM Radio options.
It also was not available with the Rear Spoiler equipment.

Thanks for the info Vintage 68. My car does have a rear spoiler, but also the rear antenna. Sounds like I may have a "non-original" mix. What are your thoughts

Pretty much what yours are...

One or the other is probably not original.

Lots of these spoilers where sold 'over-the-counter' and dealer or owner installed, even on new cars, in 1968. The spoiler was first offered in 1967 but, only over the counter. 1968 was the first year it could be ordered installed at the factory, but it took the dealers and customers some time to get in the groove of doing this and most early cars I've seen didn't have them.
If Rear Antenna is of the correct design, is installed in the right location, the cable to it is installed in the chanel and under the dash with the proper clips along it's length and the vehicle originally had an AM Radio then there is a greater chance the antenna may be original than the spoiler - IMHO of course.

Hope others chime-in with input!