Top tethering a forward facing harnessed carseat is mandatory if you happen to be in Canada, and optional if you're in the U.S. and using a carseat that doesn't specifically require it. I don't believe your TrueFit requires top tethering, but it is highly recommended to top tether forward facing harnessed carseat if the vehicle is equipped with an appropriate top tether anchor because tethering reduces head excursion, the amount the child's head moves forward/beyond the carseat's protective shell during a crash.
I don't see the Cavalier convertible model listed as an exclusion from being retrofittable in the LATCH manual's Chevrolet section, which is encouraging for you.
The only excluded Chevrolet models listed are the pre-1993 Chevy Camaro, and the Chevy Corvette (all Corvettes). You should definitely inquire at your Chevrolet dealership to see if your Cavalier convertible can be retrofitted, and also double check with another Chevy/GM dealership and GM corporate (tel. 800-222-1020) if the first one you contact says no to be verify about any possibility of retrofitting. I can't give you a tether anchor retrofit part number for your car from the LATCH manual, though, because the LATCH manual doesn't include any part numbers for retrofittable GM models as the part numbers tend to change over time, and GM dealerships are supposed to have the current tether anchor retrofit part information. I can however give you the pertinent GM Technical Service bulletin number regarding 1 free top tether anchor retrofit in retrofittable GM models in the U.S. that you'll need to refer the Chevy/GM dealership to: GM Technical Service Bulletin Number 99-09-40-004a dated April 12, 2005. Good luck and fingers crossed you can get the car retrofitted.