I dont know what to look for anymore to fix my car

I have a 1989 mercury tracer with 191,000 miles on it. I bought it a month ago and it ran fine for 3 weeks with no problems but every since the guy I bought it from put a deck in it, it breaks down. I can buy a new battery and it will be drained in a few hours and it will be completely dead. I got a new alternator put in and changed my cables to the battery but it still dies. I took the deck out and it is still dying. It runs better in the day but by night when I use the lights it starts to die when I press the brake thats when it completely dies and it wont start back up until I put a new battery in and then the process starts all over again. I dont know much about cars and I need help to figure out what the problem is before I spend alot of more money to get something fixed and find out its still not what is wrong. Please help! Thank you.