That oval cover is the plug, it is sealed with silicone, once you remove the cover the new one goes in the hole and a new bracket goes over the flange of the heater to hold it in place. The Toyota heater comes with the heater, bracket and new bolts with shake-proof washers. If you grind off the rounded nut, you'll have to drill and extract the stud in order to install the new one. If possible, use a fluted socket specifically for removing stripped nuts, or hammer on a 12 point socket that is slightly smaller than the nut size, because drilling and extracting that broken stud is a real pain.
If you do get the nuts off, the proper way to do it is to remove the studs from the block with a reverse torx socket, stud remover, or double nut. The new bolts that come with the heater will bolt the new bracket in place.
so the heater i have wont work?
Not likely if it didn't some with the hold down bracket
is there another plug behind the cover?
No, the cover is the plug, and it's not very deep behind that cover, so most conventional style block heaters where the element sticks out past the heater body will not fit.
There's no press in style plug in the aluminum blocks, they wouldn't stay in.
ok thank you
i appreciate it
oh, can the nut be heated or will it melt the silicon?
It will probably melt the silicone, but a bigger problem would be melting the aluminum holding the studs in if you use too much heat. You're going to remove the silicone anyway once you get the cover off.