Your battery isn't being charged or accepting a charge
that light usually means that the battery is going bad or that your alternator isn't charging anymore
Disconnect battery, if light comes back on you need to get the code read and repair the error.
Check fusable links
The battery light comes on when the alternator is either over or under charging. If the alternator tested good you have a wiring or computer problem. The engine computer is the voltage regulator.
The owner's manual shows that the 2000 Ford Ranger , all available engines , came with Ford Motorcraft BXT - 59 batteries
The battery light stays on when the alternator is over or under charging.
The starter solenoid should be located on the inside fender close to the battery. It will have the positive battery cable connected to it.
Any place that sells batteries can tell you the correct size to buy.
altinater is probobly dead
The OFF light also serves as a check light. When you first start the truck, the OFF light illuminates as it checks, then turns off when it senses the airbag is on.
When the windshield wiper on a 2000 Ford Ranger comes on by itself this can signal a damaged windshield wiper switch. Changing these switches out often corrects this problem.
First, you need to fix whatever's wrong, then you unhook your battery cable for about 30 minutes, then re-hook. If it comes back on, you have a problem that was not addressed.