2004 Cadillac SRX electrical system problems

My 2004 cad. Srx showed a left rear flat tire. I pulled the vehicle to a level, asphalted, scenic overlook area, 23 miles from page, az. I could not get the shifter from drive into park gear, nor would the key turn or release from the ignition. I got out to check and change tire, spare would not release, either. I called on-star, gave emergency details, to first advisor, then advance diagnostics.

Every time I start my car I get service brake assist and service stability sys" and I called my mechanic and he said he has read a lot of complaints about this. After researching I did not want to go spend a lot of time and money since it seams no one has the answer to our problem. My car seams to run good it is just scary to think there might be something really wrong with my car. I hope you guys can get the answer to our problem [xxx]