Leo,
Take a multi meter and check the battery cold. It should be over 12.0v
If it is lower, the battery is an issue.
As mentioned, check all connections.
After that, run the rig a bit and see what the actual battery voltage is. Running, it should be at least 13.7. Right after shutting it down it should read about the same. You can have a shop do an amp draw/load test on it also. That will tell ya what shape it in.
Danny

1985 SR5 4Runner
eLocked with Carter Lock
4.7:1 single case
Davez Offroad twin stick