Uno de los faros sigue sin funcionar bien, y la mecánica dice que tendrá que cambiarlo mañana.

Because uno de + plural noun is the normal Spanish pattern for one of the ....

So:

• uno = one
• de los faros = of the headlights

Together: uno de los faros = one of the headlights

Even though uno is singular, the noun after de stays plural because you are talking about a group:

• uno de los libros = one of the books
• una de las puertas = one of the doors

So this structure is completely normal.

Yes, faro can mean lighthouse, but in everyday Spanish it can also mean a headlight on a vehicle.

In this sentence, because we are talking about a mechanic and changing it, faros clearly means headlights.

This is very common in Spain Spanish. In car contexts:

• el faro delantero = front headlight
• el faro trasero is less common for the rear light, because people more often say piloto for rear lights

So here uno de los faros most naturally means one of the headlights.

This is a very useful Spanish structure:

• seguir + sin + infinitive

It means to continue / to still not do something.

So:

• sigue = continues / still
• sin funcionar = without working
• bien = well / properly

Together:

• sigue sin funcionar bien = it still isn’t working properly

More examples:

• Sigue sin llegar. = He/She still hasn’t arrived.
• Sigo sin entenderlo. = I still don’t understand it.
• Seguimos sin noticias. = We still have no news.

This is a very common structure in Spanish.

Because Spanish often uses seguir sin + infinitive as a fixed pattern.

Compare:

• No funciona bien. = It doesn’t work well.
• Sigue sin funcionar bien. = It still isn’t working well / It continues not to work properly.

So sin funcionar is not random here. It belongs to the pattern seguir sin + infinitive.

You could say uno de los faros no funciona bien, but that would simply say it doesn’t work properly.
Using sigue sin funcionar bien adds the idea that this problem is continuing.

Here la mecánica means the female mechanic.

In Spanish:

• el mecánico = the male mechanic
• la mecánica = the female mechanic

So this sentence is referring to a woman.

Be careful: la mecánica can also mean mechanics as a field, depending on context, for example:

• Estudia mecánica. = He/She studies mechanics.

But in this sentence, because it says dice que tendrá que cambiarlo mañana, it clearly refers to a person: the mechanic says...

This is a combination of two common parts:

• dice que = says that
• tendrá que = will have to

The verb tener que + infinitive means to have to do something.

So:

• tiene que cambiarlo = has to change it
• tendrá que cambiarlo = will have to change it

In the sentence:

• la mecánica dice que tendrá que cambiarlo mañana
• the mechanic says that she will have to change it tomorrow

This future tense often sounds natural when someone is giving a professional assessment or saying what will need to happen next.

Both are possible, but they are slightly different.

• tendrá que cambiarlo mañana = she will have to change it tomorrow
• tiene que cambiarlo mañana = she has to change it tomorrow

The future can sound a bit more like a conclusion, prediction, or planned necessity based on the situation.

A mechanic examining the car might naturally say:

• Habrá que cambiarlo
• Tendré que cambiarlo
• Tendrá que cambiarlo

These all suggest that, given the problem, replacement will be necessary.

So the future here is very natural.

Cambiarlo is made of:

• cambiar = to change / replace
• lo = it

So cambiarlo = to change it or, more naturally here, to replace it

In car-repair contexts, cambiar often means replace, not just change in a general sense.

So here:

• tendrá que cambiarlo = she will have to replace it

Because lo refers back to uno de los faros.

Even though faros is plural in the phrase uno de los faros, the real thing being referred to is uno — one headlight — so the reference is singular.

And because faro is a masculine noun, the pronoun is masculine singular:

• el faro → lo

So:

• cambiar el faro = to replace the headlight
• cambiarlo = to replace it

If it referred to a feminine singular noun, you would usually expect la instead.

Because cambiar takes a direct object, and Spanish direct object pronouns are:

• lo / la / los / las

Since the thing being replaced is the headlight, that is a direct object, so lo is correct.

• cambiarlo = replace it

Le is normally an indirect object pronoun, so it would not be standard here.

Bien means well or properly.

So:

• funcionar = to work / function
• funcionar bien = to work well / work properly

This tells us the headlight is not necessarily completely dead; it may simply not be working correctly.

Compare:

• no funciona = it doesn’t work
• no funciona bien = it doesn’t work properly

That little bien makes the description more specific.

Because decir que normally introduces reported information, not doubt or unreality.

So:

• dice que tendrá que cambiarlo = she says that she will have to replace it

This is a straightforward statement, so Spanish uses the indicative.

You would expect the subjunctive after expressions involving doubt, emotion, desire, etc., but not here.

In normal reading, mañana goes with cambiarlo:

• tendrá que cambiarlo mañana = she will have to replace it tomorrow

That is the most natural interpretation.

Technically, context always helps decide exactly what a time word modifies, but here the intended meaning is clearly that the replacement is for tomorrow, not that the speaking happens tomorrow.