What can drain your monte carlo car battery?
Not charging (bad alt) or it has a draw. Something is drawing power which can be anything that takes electricity or the plates in batt are worn out. test it for voltage, then check alt, if alt light not on, search test vehicle elec for current draw to see procedure.
78-87 Monte Carlo is a G-Body GM car that is smaller than a 77-90 Caprice which is a B-Body GM car. The seats in a Monte Carlo will fit in a Caprice, but they may not look quite right. You will need to do bolt location modifications. I would not recommend trying to use a bench seat from a Monte in a Caprice. It is not wide enough.
A starter does drain a battery. The battery holds a charge so that you can use your starter to start the car. After the car is started the alternator recharges your battery and runs your car and all it's accessories. If your car won't start or is having a hard time starting you can drain your battery by repeated unsuccessful attempts to start it.
The basic method of draining the coolant is the same in all cars. Basically there should be a drain plug at the lower part of the radiator. Look in the car handbook to locate this. Open this and keep a bucket or container as it will drain fluid. Once drained, close the drain plug and top it with coolant. Check the ratio of the coolant mixture on the bottle