Assuming the SX4 is built like my 1984 Sportwagon, you cannot reach the firewall bolts from under the car.
I had to remove the left front wheel and unbolt about half of the fender lining. I could then peel it back enough to reach the parking brake nuts.
Incidentally, if you find a source for the parking brake pedal assembly, I would like to hear about it. I spent many hours getting to mine and psuedo-fixing it. The fix lasted about three days (just long enough to get my Texas Safety Inspection). I'd like to just replace it.
Hope that helps, and good luck.
John B