Okay, this is a common problem with recessed lighting and fixtures, especially those with short pigtails. Here's a breakdown of your options, along with their pros, cons, and considerations, keeping safety and code compliance as paramount

Understanding the Constraints:
Code Compliance

You're right to be concerned about code. You cannot bury electrical connections inside walls without accessible junction boxes. This is crucial for safety (fire hazard, maintenance). Additionally, altering UL listed appliances can void the warranty and insurance coverage in case of a fire.

Mirror Cabinets

Working inside pre-manufactured devices can be risky, but not impossible. You need to be mindful of clearances and safety standards.

Chinese Manufacturing

While generalizations are unfair, some electrical standards and wire gauges in foreign-made products can be different. It's important to be extra cautious and verify compatibility.

Your Options (in order of preference, safest to least safe)


The Ideal: External Extension to a Properly Accessible Junction Box (Recommended)



How it works

This is the best and safest way. This involves mounting a small, shallow (pancake) junction box above the mirror cabinets. Run conduit from the mirror cabinet up to the junction box for each cabinet. Connect the factory pigtails to new conductors inside the junction box using appropriate wire connectors (Wago, etc). Extend the conductors down to the vanity light box.

Pros



Meets Code

The connection is accessible in a proper junction box.

Safest

No modifications to the mirror cabinets themselves.

Easiest to Inspect

Easily inspectable by an electrician or inspector.

Flexibility

Easy to change wiring in the future if needed.

Cons



Aesthetics

Requires a small junction box to be visible above the mirrors. (Choose a color that blends with the wall, or see if you can find a very shallow box.)

Labor

Requires a bit more effort to install the junction boxes and conduit.

Important Notes



Box Size

Ensure the junction box is appropriately sized for the number of wires being connected. Follow the fill calculations in the NEC (National Electrical Code).

Conduit

Use appropriate flexible conduit (like liquidtight flexible metal conduit, LFMC) to protect the wiring from the mirror cabinet to the junction box.

Grounding

Make sure to properly ground the junction box.2.

External Extension Using Low-Voltage Extension Cord (Alternative to using an external J-box and conduit)



How it works

It involves cutting the factory pigtails and install male/female barrel connectors that can be run from the light to the vanity outlet, the barrel connectors should be rated for the lights voltage and wattage.

Pros



Cleaner Look

Potentially cleaner look, though this may be subjective.

Easier to Remove Lights Lights are easily removed for repairs. Cheap Extension cords are cheap, and you do not need a certified electrician to perform this. Cons



UL Listed

Any modifications voids UL listing of the light, and may not be compliant with code.

Not Safe

Splicing wire and connecting it to an extension cord that is behind drywall is unsafe and illegal, and you should expect an electrician to fail you if you use this setup.

Important Notes



Gauge

Pay special attention to the wire gauge of the factory pigtails. Use extension cords of the same gauge or larger (lower number).

Polarity

Make sure you connect the wires correctly (positive to positive, negative to negative). If the lights flicker you may need to swap them.3.

Extending Pigtails Inside the Mirror Cabinet (Proceed with Extreme Caution)



How it works

Carefully open the mirror cabinet's electrical compartment. Identify the wire gauge and type used internally. Purchase the exact same gauge and type of wire (THHN is common). Purchase high-quality wire connectors (Wago or similar push-in connectors, not cheap wire nuts). Disconnect the existing pigtails (document everything!). Extend the wires with the new, longer wires and connectors. Ensure all connections are mechanically sound and electrically insulated.

Pros



Cleanest Look

Completely hidden wiring.

Cons



Risk of Damaging the Cabinet

You could damage delicate components during disassembly.

Voiding UL Listing

Modifying the internal wiring most likely voids the UL listing.

Potential for Problems

Tight spaces, unfamiliar wiring configurations, and potential for future problems if the connections aren't perfect.

Heat

If you add new internal wiring, be sure that it won't be subjected to any heat exposure within the light fixture. This can degrade the insulation and lead to shorts.

Important Notes



Research First

Before opening the cabinet, try to find any documentation or diagrams for the wiring.

Disconnect Power

Always disconnect the power to the circuit before working on any electrical wiring.

Photos and Documentation

Take photos of the wiring before you start disconnecting anything.

Strain Relief

Ensure the new pigtails have adequate strain relief where they exit the cabinet.

Heat Dissipation

Adding wire connectors inside the fixture could add more heat to the internal electronics.

Check with the Manufacturer

If possible, reach out to the manufacturer to see if they can provide longer pigtails or guidance on extending them safely.

Before Proceeding, Strongly Consider



Consult a Licensed Electrician

The best advice is to consult a qualified, licensed electrician. They can assess the situation, determine the safest and most code-compliant solution, and perform the work correctly. This may cost more upfront, but it can save you time, money, and potential safety hazards in the long run.

Inspect the Mirror Cabinets Thoroughly

Before you start any work, carefully inspect the mirror cabinets for any signs of damage or poor workmanship. If you find anything concerning, return them.

Important Considerations for ANY Solution



Wire Gauge

Use wire of the same gauge (or larger) as the existing wiring.

Wire Type

Use the correct wire type (THHN, NM-B, etc.) for the application.

Grounding

Ensure a proper ground connection is maintained throughout the entire circuit.

Strain Relief

Provide adequate strain relief where the wiring enters and exits the cabinets and junction boxes.*

Local Codes: Always comply with all applicable local electrical codes.In summary, the safest and most recommended approach is to use external junction boxes and conduit to extend the wiring. Modifying the internal wiring of the mirror cabinets should be a last resort, and only attempted if you are extremely comfortable with electrical work and understand the risks involved. And again, consulting a licensed electrician is always the best course of action. Good luck! Flag for review