The brushes on an alternator carry current.
A bad belt, a bad regulator in the alternator or bad brushes in the alternator.
Change alternator or check alternator brushes
Stator, armature, rectifier, bearings, brushes,
Is it possibly a broken/missing alternator or serpentine belt? (nope just alternator light) It can also be a failing voltage regulator within the alternator, or a failing alternator itself. It is usually the carbon brushes in the alternator when both lights come on, it indicates a field problem with alternator Brushes are not to hard to change on a toyota, just remove the alternator from car, remove rear cover unscrew the brush holder and rectifier?(they are attached together) and install new brushes... you will need a soldering iron. It is a simple repair and at less than 10$ is more affordable than replacing the alternator. VW and Volvo have removable brush holders that unscrew from back of alternator Gm Ford and Mopar, you may have to split alternator to change brushes, there is often a small hole in back of alternator where you can use a piece of wire to keep brushes compressed when you put alternator back together easy job ... save your money
It would be best to remove the alternator and take it to an electrical shop.
Regulator is mounted on the back of alternator. It is replaceable if you can find parts. Brushes are there also.
A failing alternator with sticky or worn out brushes.
The brush holder in an alternator keeps the brushes held to slip rings with spring pressure (or commutators in generators or some motors) which provide electrical contact to electromagnets in the spinning rotor. By varying the electrical current through the brushes to the rotor magnets, you can adjust the power output of the alternator as needed.
certainly, sand is very abrasive and can ruin the bearings, windings, and brushes.
Field coils and armature
In an alternator, brushes are conductive components that maintain electrical contact with the rotating part called the rotor. They transfer current from the stationary part of the alternator to the rotor, allowing it to generate a magnetic field. This magnetic field is essential for inducing alternating current (AC) in the stator windings, which is then converted to usable electrical power for the vehicle's electrical system. Proper functioning of the brushes is crucial for the alternator's efficiency and overall performance.
Sounds like the brushes are worn down in the alternator.