EASY TO CONNECT: No instruction manual, so you must note that the brass pin on the switch must be connected to the negative for its backlight LED, whereas the two nickel pins are the positive-from-battery and the positive-to-load EASY TO INSTALL: Not have much spare clearance, when the 4 mounting holes are large enough to accommodate the device, they are very nearly the same size as the cover plate. So, measure and drill VERY CAREFULLY lest your cutout becomes larger than the plate meant to cover it! 4 IN 1 DESIGN: Linkstyle multifunctional 12v switch panel designed with RV USB outlet 12v + LED digital display voltmeter+ 12v cigarette lighter outlet + LED rocker toggle switch. Convenient for installation and operation. Also detachable to replace other charger sockets or anything else unit that fits. DUAL 12V USB CHARGER OUTLET: This 12V USB socket is built-in dual USB charging ports, 2 ports output: 5V / 2.1A & 2.1A (total 4.2A), will auto-detect your devices and simultaneously charge Features: 1. Wide voltage design (12-24V wide voltage design, universal for 12-24V models, applicable to various standard on-board conversion plugs) 2. USB support (2 USB interfaces that meet the needs of various electronic devices, suitable for mobile phones, IPADs, digital cameras, etc.) 3. Streamlined design (with switch and waterproof cap, compact and exquisite design, easy installation, complementing your car) 4. Fast charging (intelligent power supply IC, stable current output without fluctuations; low resistance wire, strong current conductivity, safe and stable) 5. Five intelligent protections (overcurrent protection, voltage stabilization protection, overcharging protection, overvoltage protection, and short-circuit protection) 6. High brightness LED light (three digit Nixie tube display, constantly monitoring the battery voltage, making the battery work under normal voltage, extending and protecting the service life of the battery and the engine)