It looks like front to back space is pretty tight, and I assume there are no tether anchors? It will definitely be a challenge.
At what age do you expect to start driving that grandchild around?
The general strategy should be to find a very compact, long lasting rear facing seat, then around age 5.5-6, switch to a belt positioning booster.
For newborns without head control (so they need to be pretty reclined, which takes up more front to back space) you’ll either have to give up all your passenger side leg room or find a car seat that fits in the center between the front seats.
For the newborn stage, I would look at:
Chicco Keyfit
Graco SnugRide SnugLock 35 DLX
Combi Coccoro (doesn’t come out of the car between trips—you just buckle the baby in)
Cosco Scenera NEXT (also stays in the car)
For long term rear facing, unless something new comes out in the meantime, you probably want a Graco Extend2Fit.
Once you hit booster age, come on back and tell us about the kid’s height and weight and we’ll probably recommend something that isn’t yet on the market.