The Elantra was redesigned for 2017 – and unfortunately now has rigid buckles for 2D & 2P that are flush with the seat cushion… which will make fitting 2 car seats side by side or 3-across more of a challenge as even our narrow booster no longer fit in 2C with this redesign. 
The Elantra was redesigned for 2007 – and now has a head restraint for 2C, in addition to those for 2D & 2P. There are flexible buckles for all 3 seats in back.
The Elantra was redesigned for 2001. It now for the first time has lower anchors (in 2D & 2P), a shoulder belt for 2C, and head restraints for 2D & 2P. There is still no head restraint in 2C.
The 2000 back seat is identical to the 1999 – except starting for 2000 there are tether anchors for all 3 seats (no need to retrofit anymore!).
Tether anchors can be retrofitted in these 1992-1999 vehicles – there are weld nuts in the shelf below the rear windshield specifically for a tether anchor hardware to be screwed into. Check your glove box for the tether anchor retrofit kit. This model year range has a lap belt in 2C and no head restraint in 2C. The vehicle seat back is slightly higher in 2D & 2P but it is likely not a sufficient head restraint for an adult.
