Tekonsha 118578 ModuLite 4-Way Flat Harness | T-One
Tekonsha 118578 ModuLite 4-Way Flat Harness | T-One — Product description
The Tekonsha 118578 is the plug-in fix when your OEM 4-way flat tow harness has had enough. It's a T-One connector paired with an upgraded, circuit-protected ModuLite module, so your trailer lights pull power straight from the battery instead of the vehicle's factory circuits. Clean signal, protected wiring, no cutting.
If you tow a boat trailer, you know the factory 4-way flat is often the first thing to give out. The Tekonsha 118578 is the replacement, and it's a T-One connector, which means it plugs straight into your vehicle's existing wiring harness. No splicing. No scotch-locks. No chasing a bad ground six months later.
Here's what makes this one worth the money. The upgraded ModuLite module is a powered converter, not a simple splice-in tap. It pulls the current your trailer lights need directly from the battery, so your vehicle's factory light circuits never see the extra load. That matters on newer rigs where the taillight computer will throw a fault, or just quietly stop working, if you overload it. The module takes the hit instead.
And it's circuit protected. If something on the trailer side shorts out, a corroded connector, a pinched wire, a bulb that finally lets go, the module protects the run rather than passing the fault back up into your vehicle's wiring. That's the upgraded part of the name doing real work.
Installation is about as painless as trailer wiring gets. You find the factory harness connector, unplug it, plug the T-One in between, and route the 4-way flat back to the hitch. Most folks are done in well under an hour with basic hand tools. Because it's a direct-fit T-connector, you're not guessing at wire colors or reading a factory diagram.
One thing to check before you order: fitment. This is a vehicle-specific T-One, so confirm the 118578 part number matches your exact year, make, and model before you buy. If you're not sure, send us the vehicle details and we'll tell you straight whether this is the right connector for your setup. We'd rather get you the right part the first time than have you fighting a harness that won't seat.
For a boat trailer running standard 4-way flat lighting, this is the clean, protected way to wire it up. Plug in, protect your vehicle's circuits, and get back to the ramp.
Key Features
- 4-Way Flat output connector, the standard for boat trailers and small utility trailers
- T-One plug-in design connects to your OEM harness with no splicing or cutting
- Upgraded circuit-protected ModuLite module isolates your vehicle's factory light circuits from trailer-side faults
- Powered converter draws trailer lighting current from the battery, so newer taillight circuits don't overload
- Direct replacement for a worn or failed OEM tow wiring harness
- Vehicle-specific fit, installs in under an hour with basic hand tools
Why Buy from NVN Marine
- Authorized Tekonsha reseller, full manufacturer warranty
- NMEA member and ABYC certified, advice from real boat techs
- Same-day shipping before 3 PM ET on in-stock items
- NY headquarters and Fort Lauderdale flagship retail store
Technical specifications
| Title | 4-Way Flat Replacement For OEM Wiring Harness With Upgraded Circuit Protected ModuLite Module |
|---|---|
| Brand | Tekonsha |
| Manufacturer Part Number | 118578 |
| UPC | 016118121667 |
| Connector Type | 4-Way Flat |
| Module | Circuit Protected ModuLite |
| Connection Type | T-One plug-in connector |
| Power Source | Vehicle battery (powered converter) |
| Circuit Protection | Built into the ModuLite module |
| Function | Direct replacement for OEM 4-way flat wiring harness |
Frequently asked questions
What does the ModuLite module actually do?
It's a powered converter. Instead of tapping your vehicle's factory light circuits, it pulls the trailer lighting current straight from the battery. That keeps the extra load off your taillight wiring, which matters a lot on newer vehicles that fault when overloaded.
Do I need to cut or splice any wires?
No. This is a T-One connector, so it plugs into your vehicle's existing harness. You unplug the factory connector, plug the T-One in between, and run the 4-way flat to the hitch. No scotch-locks, no wire cutting.
Will this fit my vehicle?
The T-One is vehicle-specific, so confirm the 118578 part number against your exact year, make, and model before ordering. If you're not sure, send us the vehicle details and we'll check it for you.
What kind of trailer plug does it give me?
A standard 4-way flat connector, which is what most boat trailers and small utility trailers use for running lights, brake, and turn signals.
What does circuit protected mean here?
If something shorts on the trailer side, a corroded pin, a pinched wire, a failed bulb, the module protects the circuit instead of passing that fault back into your vehicle's wiring. It's the safeguard that saves you an expensive repair up front.
How long does the install take?
Most people finish in well under an hour with basic hand tools. Because it's a direct-fit plug-in, you're not reading a factory wiring diagram or matching wire colors.
