[MA70-3.0GT]

there is always the possibility that the batteries are not being "installed" properly too. When you buy a new battery & they fill it with acid, just because it can start the car straight off on it's "dry-charge" that it left the factory with, doesn't mean it's ready to use.

A freshly filled dry-charged battery should ALWAYS be put on a trickle charger and left to fully charge, ideally overnight on an automatic device. Otherwise the battery never reaches it's full power potential & can sulphate up very quickly also.

Not saying this is your problem, and I would definitely advise checking for draw as described (although this is a general test and could be alternator diodes/stuck relay/door switch/faulty alarm or anything really) and rule out any issues before replacing another battery.

Also, there are fusible links in the battery wiring that can corrode (the one that causes charging probs is marked ALT I believe) but I'm assuming you checked/replaced withthe wiring?

EDIT-:When the car is started & running, you could also put the multimeter across the battery terminals and read the voltage, this should be between 13-15v ideally if you're charging properly (do this after starting the car with a fully charged battery thats been off the charger for at least an hour & is showing around 12.7 volts disconnected) and remember overcharging (15.5v plus) can kill batteries just as well as undercharging.