I just looked at mine, its visible because my car is a little cut up. I'll try to get a pic up here later. It is a captured plate, not very thick, not thick enough to use a heli-coil.
You need to try real hard not to break any more.
To gain access you will probably have to pull the fuel tank and cut an access hole in the body support that runs side to side.
With the tank back in you should not be able to see the access holes.
Work your remaining bolts out with pb blaster, lots of it, turn the bolts back and forth and add more PB. If you look under the car, there may be a place you can spray some PB on the back side.
If you can get the remaining bolts out, find someone that is talented with a drill, drill out the broken bolt and re-tap the hole. Use anti-seize on the threads when you go back together.