To identify the correct CURT trailer plug adapter, record the original vehicle-side socket and trailer-side plug rather than relying only on the number of contacts. A six-round plug may have different centre-pin functions, and a four-flat adapter cannot supply electric-brake wiring by itself.
Start with the two connector ends
4-way flatBasic running, stop and turn lighting, plus ground. Electric trailer brakes and auxiliary power require additional circuits.
6-way roundRound pin layout. The centre pin may carry different functions depending on the trailer's wiring; match the exact pinout.
7-way RV bladeSeven blade-style contacts for standard trailer lighting plus correctly wired extra functions such as brakes, backup and auxiliary power.
Do not assume every adapter carries all functions. When the vehicle starts with only a 4-flat, adding a 7-way socket does not automatically install a brake controller, auxiliary power or reverse circuit. Match the vehicle harness and trailer requirements.
Identify an adapter without reversing its function
- Photograph the old vehicle-side socket and trailer plug, and count their actual contact shapes.
- Check whether the original was 7-way RV blade, 6-way round or a 4-flat connection.
- For 6-round hardware, compare centre-pin function rather than assuming all six-round connectors are alike.
- Distinguish a plug-in adapter from a wired mounting kit with tester or added outputs.
- Test ground, tail, turn, stop and any required brake/auxiliary circuits after installing the properly matched adapter.
Compare the exact vehicle socket and trailer end
| MPN | Vehicle input | Trailer output | Key distinction |
|---|---|---|---|
| 57004 | 7-way RV blade | 4- or 5-way flat | LED-recognition adapter |
| 57660 | 7-way RV blade | 6-way round | 7-blade to 6-round adapter |
| 57621 | 6-way round | 4-way flat | 6-round to 4-flat adapter |
| 57674 | 4-way flat | 4-flat plus 7-blade | Dual output with tester |
Connector descriptions show nominal shapes and intended directions. Check the current vehicle and trailer wiring diagrams before connecting.
Find the exact connector adapter listings
Compare connector shape and manufacturer number, then check the product's condition and availability.
7-blade vehicle socket outputs
CURT 57004 — LED-recognition adapterBuilt-in load resistors for compatible LED trailer diagnostic systems. A 4-flat does not supply electric trailer brakes.Check listing and availability
6-round connections
CURT 57660 — 7-blade to 6-round adapterCURT identifies the 6-round center pin as the electric-brake contact. Check the trailer's center-pin wiring.Check listing and availability
CURT 57621 — 6-round to 4-flat adapterConnects a six-round vehicle socket to a four-flat trailer plug for tail, brake and turn lights; it does not add electric brakes to a 4-flat.Check listing and availability
4-flat vehicle inputs and added outputs
CURT 57674 — Dual output with testerDual output and tester. Four-flat supplies basic lighting only; other circuits need separate wiring as applicable.Check listing and availability
UsedRVParts offers viewing and pickup by appointment in Kelowna. Check each listing for stated condition.