I need to know where to find the original code to reset the key less pad on a 2000 Mercury Mountaineer

look on the back of the computer, it is under the dash or kick panel. you can add your own code but cant change the original

it may also be in the rear panel behind the water reservoir

There is nothing to reset you can not reset the original code so what exactly do you want to do there is nothing about the system that you can change no more then create your own code but the original code will still exist.

Open the compartment in the back on the drivers side where the jack tools are. If u look inside towards the front of the vehicle the module will be behind the panel just a little ways past the opening for the cover. Get a flashlight to shine in to see the module. You may need to pry the trim piece back a little in order to see the front of the module (the side that is facing out or away from the body of the vehicle). There will be a white decal with the factory 5 digit code on it on the module. I've gotten many codes doing this. Good luck.