Yeah. First job is to see if there is any wiring for them. If there is, then it’s a simple enough job to get the door cards off and wire the speakers up.
I haven’t looked for many years, but all the usual car audio suspects (Kenwood, Pioneer, Alpine etc) had speaker selectors where you put your car in and it told you what speakers would fit as direct replacements, which would make choosing them nice and simple.
VW are particularly special at scrimping on stuff like this – yeah, lovely quality dashboard and the doors make a nice heavy thunk, but we won’t bother with rear speakers as we have to save money somewhere. My sister’s 05 Polo came with a cassette player as standard – on a brand new car in 2005!