Welcome to car-seat.org. Using the lower LATCH anchors/UAS is an alternate installation method in a car equipped with LATCH/UAS, but the vehicle seatbelt is always an allowable install method. You'll need to use the vehicle seatbelt to install the carseat in the Impala following the directions of both your carseat's instructions and your vehicle's manual for installing with the vehicle seatbelt.
You'll need to determine how the vehicle seatbelt locks for securing a carseat installation and this link can guide you through determining the seatbelt type and how it locks:
http://carseatsite.com/lockingclips.htm There are some helpful installation videos online for various types of carseats at
https://www.youtube.com/user/CarSeatSite/videos that you may find helpful, and you can always post back here with the manufacturer and model name of the carseat you own for more installation pointers.
If you wish to have in person assistance from a certified child passenger safety technician, you can check the listings of car-seat.org members who are certified and willing to help out in person at
http://www.car-seat.org/showthread.php?t=67767 to see if there's a Canadian tech in your area to contact via PM (private message).
Also, if per chance you're installing a forward facing harnessed carseat in the Impala, in addition to using the vehicle seatbelt to install, you'll also need to use the carseat's top tether because top tethering a forward facing harnessed carseat is mandatory in Canada (and recommended beyond Canada due to its important safety role in reducing head excursion during a crash). Since I believe your Impala pre-dates having factory equipped top tether anchors, you'll need to contact a Chevy/GM dealership to arrange retrofitting top tether anchors if/when you need to install a forward facing harnessed carseat.
