How do you replace battery in 1999 Monte Carlo?

First make sure the ignition is in the OFF position and the key is removed. You will need a (13mm) wrench or socket to remove the bolts on the driver's side cross brace. If they are rusted spray them with PB blaster or the equivalent and wait 10 or 15 minutes and try again, repeat if necessary. (WARNING do not force the bolts or they may break!) Remove the cross brace. Disconnect the sensor on the washer fluid reservoir and carefully lift it up. (WARNING be careful not to pull the hose loose or you'll have a mess!) The battery is here. Disconnect the negative (ground) cable from the side post first (?mm), making sure it does not touch anything metal and complete the circuit again. (WARNING This could cause electric shock to you or damage your electrical system!) Then disconnect the positive cable from the side post (?mm). Remove the plastic cap over the auxiliary post and use a (9mm or 10mm?) socket to remove the nut and then remove the auxiliary cable. Then there should be some clips holding the battery in place, carefully release them, you don't want to break them. Reverse this process for installation. It is important to remember to connect the negative last because it will complete the circuit. (See above WARNING electric shock/damage) You may need to reset your idle code (simple process) and/or unlock your radio with your LOC code.

I am doing a battery swap on my 99 Monte Carlo later today and will add any updates such as other tools required and updated socket sizes.

This information is also relevant to the 97-00 Chevy Monte Carlos and Luminas.