First question is did both doors locks start having this problem at the same time? If so that tells you a lot. If no, that also tells you a lot.
Both going bad at once is pretty much a wiring issue.
The odds of all 3 switches failing at the same time is minute.
Even with a bad door switch, if the wiring and actuators at each door are still good, the master switch should still operate them.
In many cases, when the master switch fails, it doesn't fail completely, and one of the door locks could still be working.
With the age of the vehicle, if one lock went bad at a time its more likely to be the actuators themselves or a bad power or bad ground wire connection, and after those, the switch-as switches usually live in a better protected-from moisture/rust location. Of course if someone removed and didn't replace water shields in the doors, and/orthe window glass outside weatherstripping is bad, all bets are off.
Only way to know for sure if the actuators are bad is to go in the doors and look at the actuators and see if the rods are moving.
If not them check for power getting to them and if they have good grounds.
SOMETIMES a GOOD actuators rod will come off (and there may be a broken rod connector,) but in THAT case you would usually still be able to hear the actuator.
Good news is the actuators and switches are usually inexpensive-at places like rockauto.