Glad you found the top tether anchors, whew.
Your 9-5 predates factory equipped lower LATCH anchors and it cannot be retrofitted with lower LATCH anchors. This is true of nearly every pre-lower LATCH anchor equipped vehicle on the market - lower LATCH anchors cannot be retrofitted, while top tether anchors typically can be retrofitted in pre-factory equipped models. But fortunately your car is factory equipped with top tether anchors and won't need a tether anchor retrofit, whew.
So you'll use the vehicle seatbelt to install as you mentioned, and also use the factory equipped top tether anchor for the specific vehicle seating position where you're installing your ComfortSport forward facing. While I don't have experience installing in your specific vehicle, the seatbelt installation needs to have less than 1 inch of movement at the carseat's belt path to be deemed a proper installation before you attach the carseat's top tether strap's connector to the vehicle's top tether anchor and shorten the top tether strap to remove slack. Using the top tether for a forward facing harnessed carseat installation plays an important safety role to help reduce head excursion, the amount the child's head moves forward/beyond the carseat's protective shell during a crash. Using the top tether is legally required for forward facing harnessed carseat installations if you happen to reside in Canada, and recommended even if not legally required in other countries because of its role in reducing head excursion (hence the "whews" that your car is factory equipped with top tether anchors, given the potential for difficulty in obtaining top tether anchor retrofit parts for a Saab model with the upheaval in the Saab brand of late).
While we have you here on the forum, if your child happens to be under the ComfortSport's rear facing weight limit and has at least one inch of carseat shell above the top of the head, then you ideally would install the carseat rear facing until it's maxed out rear facing by weight or height, whichever max. is reached first, due to the tremendous safety benefits of rear facing compared to forward facing.