Just bought engine for my cadillac escalade yr 1999 and alternator and my battery wont stay charged what can be the problem

First check your battery. Make sure it is good, charged and the terminals are clean and making good contact.

How old is the battery? It you do not have a battery charger, take the battery out and have your local auto parts store or the store where you purchased the battery to charge and test it. They usually do this at no cost.

Have the battery checked if it is good the alternator needs replaced.

Check the charging relay if working. You can find it in the fuse box which is either in the engine compartment or under the dashboard on the drivers side legroom.

If the security light is not staying on, I would check for voltage at the starter, the neutral safety switch, and the key switch.

Have a free diagnostic scan performed at AutoZone.

Do you have power?
battery might be drained cause your alternator is not charging anymore, 2003, seems about time for a new alternator, those vehicles consume lots of power cause pretty much everything is electric, lots of motors and servos, so they depend a lot on a good power source from the battery. If alternator doesnt charge it, no power...

have power system checked.

Good luck.

bad battery connection tighten the wires and try again

bad wiring and anything that stays on-du a complete inspection of vehicle and have it scanned--disconnect batery at nite to save charge

Alternator OR Drive belt to the Alternator Loose

Replace the brake light switch - sticking . Hope this helps