What STL78AB Actually Is
This is a 6-inch oval red lamp with 10 LEDs, running on a 12V system, used as a combination stop, tail and turn light on trailers, heavy-duty trucks and RVs. It mounts in a standard 6-inch oval grommet and feeds through a three-wire pigtail or plug, depending on how the harness was originally built.
The housing is sealed and DOT-certified as a red stop/brake lamp, so there is no question about running it on the road. LED boards draw far less current than an incandescent bulb, which matters when you are running six or eight lamps off one trailer circuit and an old seven-way connector.
Our part is aftermarket. It is not made by, endorsed by, or authorized by any original equipment manufacturer. The numbers listed here exist so you can identify what you already have.
Why These Numbers Cross
STL78AB, Optronics STL78ABP and Peterson M423A-4 all describe the same physical lamp format: 6-inch oval, red lens, LED stop/tail/turn, 12V, grommet-mount. The mounting geometry on a 6-inch oval is an industry standard shape, so lamps from different brands seat in the same rubber grommet and the same body panel cutout.
The differences between the numbers are usually the connection method and the branding on the lens. One version may ship with a bare pigtail, another with a molded plug already attached. Electrically they do the same job on the same circuit.
Treat this cross-reference as a starting point, not a fitment promise. Pull the old lamp, read the number cast into the back of the housing or printed on the lens, and confirm the connector style before you order.
Checks Before You Order
Measure the cutout. A 6-inch oval grommet lamp will not go into a 4-inch round or a rectangular pocket without cutting sheet metal. Also check whether your trailer uses a grommet or a surface-mount bracket, because the same lamp body needs different hardware for each.
Check the connector. Count the wires and look at the plug on the harness side. Three wires means stop/turn, tail and ground. If your trailer grounds through the mounting bracket instead of a dedicated white wire, plan on adding a ground strap.
Then check the harness itself. Most lamps that come in for replacement were killed by water in the connector or a corroded ground, not by the LEDs failing. Replacing the lamp without cleaning the pins gets you a second failure in a few months.
STL78AB Cross-Reference & Interchange Numbers
These manufacturer and aftermarket numbers commonly cross-reference to STL78AB. Use them to identify the part — then confirm the exact fit by VIN or the number stamped on your old unit before you order. An interchange is a starting point, not a fitment guarantee.
| Brand / Line | Cross-Reference Numbers |
|---|---|
| Optronics | STL78ABP |
| Peterson | M423A-4 |
Frequently asked questions
Is STL78AB the same lamp as Optronics STL78ABP?
They describe the same 6-inch oval red LED stop/tail/turn lamp format and mount in the same grommet. The suffix usually indicates the connection style, such as a molded plug versus a bare pigtail. Confirm the connector on your harness before ordering, and verify against the number on the old lamp.
Will this fit where a Peterson M423A-4 was installed?
M423A-4 is the same 6-inch oval grommet-mount red LED format, so the cutout and grommet should carry over. Fit is confirmed by the number on the part you removed and by measuring the opening, not by the cross-reference alone. Check the wiring connection separately.
Why did my LED trailer light stop working when the LEDs never burn out?
Almost always moisture or corrosion at the connector, or a bad ground. Water wicks up the wire and eats the pins from the inside. Before you install a replacement, cut back to clean copper, use heat-shrink butt connectors, and dielectric grease the plug. A bad ground can also cause dim or flickering lamps.
Not sure it fits? We check before you pay.
Run your VIN and we’ll match parts to your exact truck, or call the counter — a person who knows trucks verifies fitment by OEM number before the order ships. No wrong-box returns.
More cross-reference guides
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Every part below is a Premium Quality aftermarket Direct Replacement with published OEM cross-reference numbers, a 6+ month warranty and same-business-day shipping from Tacoma, WA. Not sure it fits? Run your VIN — or call and a person who knows trucks will verify fitment before you pay.