99 cadillac seville sls battery

I recently got this car as an addition to my cars, but did not drive it for about a week. Now the car does not start. It almost started on the first try. Second and third and successive tries just don't work. The battery being low on voltage, I tried jump starting with another vehicle of mine. I could tell the voltage is ok from the info center button on the Caddy, that reads 12.4v when jump start wires are connected, however the car does not start upon ignition. The lights turn completely off for a second, then I see all the lights on the dash come on and then off (normal cycle) but starting is like almost chocked.   Could this be a battery that needs to be replaced? I cannot find a battery under the hood. Any info is appreciated. Thanks.

The battery is under the lower part of the rear seat. Grab the front edge of the lower rear seat on each side and pull upwards. Check that the battery terminals are clean and tight and that the battery ground next to it is in good shape. Use a volt meter to check the voltage of the battery.sparky

Thanks for the response sparkie. I checked the battery as you said. It was under the rear seat. The voltage is now 4.2 volts. I tried connecting another battery and disconnecting the caddy's battery to see if only the other battery could start. Not working. I called the dealer and he says he has to tow the car to his shop to check it out. I don't know him too well and hope I dont have a bad experience. Thanks for letting me know where the battery was. That was very helpful. Mark