[VAC] Re: running light problem

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

If the trailer light brake and directional lights work properly, there's
no diode issue. You don't need an adapter. What you need is a relay to
run the running lights. There's more current in the running lights on
the Airstream than on the cherokee and the cherokee's wiring and circuit
breaker seem to not be rated to handle both loads. Find a plug-in 30 amp
automotive relay (www.hosfelt.com, 800-524-6464, cat no. 45-287 $3.50
plus socket with wire leads 21-193 $2.09 or RS or an auto parts store). 
Wire its coil from the running cherokee running light circuit to ground.
Wire the NO contact on the relay (it should be labeled on the case) to
the cherokee battery through a heavy wire and circuit breaker or fuse
near the battery. Wire the trailer running lights to the Common contact
on the relay. This is the way that Ford does all the trailer circuits in
my '98 F-150 with the factory trailer option. Saves much grief. Many new
vehicles include sensing of lamp currents so the computer can tell the
driver of lamps burned out. Adding more lamps to the circuit confuses
the computer. Adding with a relay causes less confusion because the
relay coil current is ten times smaller than one lamp. Mount that relay
under the hood, not the rear fender.

Gerald J.