Where are the transmission shift solenoids "A" and "C" located?

Both of them are under the batter on the side of the transmission. Step one is to write down the anti-theft codes for the radio and navigation system. Then disconnect the battery. Now remove the intake air ducting and the battery, then the battery tray. Now remove the starter. With it out of the way you can get to the solenoids. A common issue with them is corrosion, The battery vents a bit of vapor that gets into the connections and causes problems. Remove the solenoids, replace them and reassemble. Toque the bolts as shown.
If you have access to a bi-directional scan toll you can test the solenoids with it. You can also test them once you get to them using the above removal steps. They are 12 volt parts, they should measure between 12 and 25 ohms between the two pins, and if you use a pair of jumper wires to apply battery voltage to them, they should click as they open and close.
Check the wiring close for damage.