Here are the parts you need:
Parts necessary from
1 ea. # 275-226 12VDC/30A SPST Automotive Relay, $6.49
1 roll #278-569 35-ft 10-Gauge Hookup WireBlack, $14.99
1 pkg #270-1084 25A Blade-Type Automotive Fuses, $1.99
1 ea. #270-1234 30-Amp Inline Blade-Type Fuse Holder, $2.49
1 pkg #278-1632 4" Nylon Wire Ties, $1.99
1 pkg #64-3111 12-10 Gauge Shrink Butt Connectors, $1.99
1 pkg #64-3140 Low-voltage Tap-Ins, $1.99
1 pkg #64-3120 Insulated Ring Connectors, $1.99
2 pkg #64-3137 1/4" Crimp-On Quick Disconnects, $1.99
Combo 7-pin Round/4-pin flat adapter with mounting bracket - Auto Zone $24.99
Here are detailed instructions:
If your vehicle already has a trailer battery charge wire routed from
the 7-pin connector to the engine compartment, you can skip steps 1-5.
1. Disconnect the negative cable at the battery.
2. Jack up vehicle and support using stands.
3. Connect 10 gauge wire to "Battery Charge" pin of 7-pin connector.
4. Route the 10 gauge wire under the vehicle to the engine compartment.
Use tie wraps to secure the wire and keep it away from heat and
5. Bring the 10 gauge wire up into the engine compartment near the
6. Mount the 30A relay near the battery using an existing screw, or
drill a hole for mounting.
7. Cut a 6" length of wire off the end of the 10 gauge wire.
8. Crimp a quick disconnect "female" connector to one end of the 6"
9. Crimp an ring connector to the other end of the 6" wire using a ring
with a hole large enough for the relay mounting screw to pass through.
10. Put the relay mounting screw through the ring connector on the 6"
wire and mount the relay.
11. Slide the quick disconnect connector onto the low current "ground"
lug of the 30A relay.
12. Locate the positive terminal of your battery.
13. Determine if the positive battery cable already has an unused 10-12
gauge auxiliary wire. If so, simply insert one lead on the 30A fuse
holder into the butt connector provided and crimp it. Skip to step 22.
14. Otherwise if there is no visible auxiliary wire, determine if the
battery uses top mount stud or side mount cables.
15. If it is a top mount stud, select a ring connector with a hole large
enough to clear the batter cable tightening bolt.
16. Crimp the ring connector on one lead on the 30A fuse holder.
17. Put the ring connector over the battery cable clamp bolt, re-install
the nut, and tighten sufficiently. Skip to step 22.
18. Otherwise if the battery uses side mount cables, follow the cable to
the starter solenoid. Disconnect the battery cable from the solenoid.
19. Select a ring connector with a hole large enough to clear the batter
cable stud on the solenoid.
20. Crimp the ring connector on one lead on the 30A fuse holder.
21. Put the ring connector over the solenoid stud, re-install the
battery cable and nut, and tighten sufficiently.
22. Cut a length of wire off the end of the 10 gauge wire long enough to
reach from the fuse holder to the relay. Leave enough to secure the
wire with ties.
33. Connect one end of the wire to the fuse hold using a "butt"
34. Crimp a "female" quick disconnect to the other end of the wire.
35. Slide the quick disconnect connector onto the high current +12V lug
of the 30A relay.
36. Route the 10-gauge wire from the 7-pin connector to the relay and
cut off leaving enough length to secure with tie wraps.
37. Crimp a "female" quick disconnect to the end of the wire from the
38. Slide the quick disconnect connector onto the high current negative
lug of the 30A relay (opposite side).
39. Crimp a "female" quick disconnect to one end of the remaining loose
40. Slide the quick disconnect connector onto the low current positive
lug of the 30A relay (opposite the ground lug).
Now you have done the easy part ;-).
41. Locate a source of +12V switched by the ignition key (i.e. only
"hot" when the key engine is running). For older cars you may find that
another terminal at the starter solenoid will meet this requirement.
For newer cars you can call a local automotive electrical shop, or
possibly get a wiring diagram from the library or a purchased manual.
42. Use the "Low-voltage Tapin" to connect the other end of the low
current positive wire from the 30A relay to the source of switched 12V