On the inside of the car - near the top back of the heater - two studs, one on each side, go through a bracket & are secured with nuts.
On the engine side of the firewall - Center of the firewall - lined up with where the bottom of the heater is inside - rubber grommet plug. Remove plug, reach in with socket on a short extension & replace nut on the bottom heater stud. Use some glue or something in the socket. If you drop the nut it is gone forever.
If you are lucky, the studs are still in the heater & all you have to do is put nuts on them. If the studs are missing you will have to remove the heater & add studs.
Good luck,
D