I don’t know all the answers but many have tried to place rear speakers in the rear of first generation Camaro's without any luck. I wanted to create this web page to help list out all the attempts and some good possible solutions for a convertible owner to consider.
I made this site to specifically address stereo upgrades and not stock installs.
1. The best install I have seen for rear speakers on a convertible were done on the Year One 69
Convertible project car .
Here are some select pictures of this install. Keep in mind that you will need power windows or to convert to power windows in the rear to pull this off.

2. Another option that some people have been happy with is simply running wires to the rear seating area, Then hooking up 6X9 speakers in small speaker boxes. The speakers are not mounted but can be quickly Removed for shows or if you have more than two people in the car.
3. You can install 6x9 speakers in the stock locations behind the seat back, in the convertible well area. This will work but you will not get the best sound out of the speakers because they will not be aimed at the passengers. You will also loose all sound when the top is down.
4. I am oppen to sugesstions...
Here are some pictures of other stereo options,
This is my car:

Here is a link to rear (trunk) speaker box plans for a 69 Camaro.
Here is a link to an Excel file that can be used to adjust the plans inside dementions to your sub woofer needs for air space.