Glider trailer wiring can drive you crazy! I finally managed to get
things working, and I offer the following tips in the hope that others
might benefit.

First, you must determine for both the car and the trailer exactly what
turn signal and brake light system is being used. Are there separate
turn signals and brake lights, or are the turn signals and brake lights
combined? The answer is easy if the turn signal lights are amber and
the brake lights are red - they're independent. If the turn signal
lights and brake lights are all red, though, you'll need to look
closely to see if separate bulbs or filaments are used (independent) or
whether the same bulb or filament is used (combined).

Next, you must find out if the car uses a computer. The best bet is the
manual, but another indication is if you get a message on the dash when
a bulb is out (temporarily pull a taillight to find out). If a computer
is used, you'll need to use a powered isolator to prevent possible
destruction of the computer. The isolator will only pull 150 milliamps
from the car's circuitry (too low to bother the computer), but it will
provide full battery power to the trailer (up to 10 amps per light
circuit) for maximum bulb brightness.

With the knowledge from these two steps in hand, here are the four
possibilities and their solutions:

1. If your car doesn't have a computer, and you're going to the same
type of system (independent to independent or combined to combined)
then you can simply wire the appropriate wires inside the car through a
suitable jack and then through a suitable plug to the appropriate wires
inside the trailer. I'd suggest fusing each circuit inside the car for
safety.

2. If your car doesn't have a computer, and you're going independent
to combined, then you need to use a Hoppy Model 48845 taillight
converter for the turn signal and brake light circuits. Again, I'd
suggest fusing each circuit inside the car (before the converter).

3. If your car does have a computer, and you're going independent to
combined, then you can use either a Hoppy 46365 (short proof) or a
Hoppy Model 46255 (non-short proof) powered taillight converter for the
taillight, turn signal and brake light circuits. Again, I'd suggest
fusing each circuit inside the car (but after the converter this time).

4. If your car has a computer and you're going independent to
independent, you have a problem! This is the situation I had with my
BMW and my Cobra trailer. Neither Hoppy or Reese offer a powered
trailer isolator for an independent system, but I found a powered
trailer isolator made by Kriss Motorcycle Products (
http://www.kriss.com/traileri.htm ). I wired it using a 6 pin jack for
the car and a 6 pin plug for the trailer (using only 5 wires), with the
supplied fuses installed between the isolator and the 6 pin jack. This
isolates the taillight, turn signal and brake light circuits of the car
from those of the trailer.

The Kriss isolator worked just fine with the independent system on my
Cobra trailer, but it wouldn't work with my club's trailers - because
they had combined turn signals and brake lights. I needed a taillight
converter, and it seemed like the Hoppy Model 48845 should work. When
it didn't, I experimented and found that all that was needed were three
470 ohm 1/2 watt resistors. Wiring these resistors to the 48845's left
turn input to ground, right turn input to ground, and brake light input
to ground got things working properly. Apparently the outputs on the
Kriss isolator module float until they are powered, and the Hoppy unit
expects to see ground potential on its inputs until they receive power.

Hope this saves someone time and trouble!

-John

P.S. - During this project I learned that some cars use what is termed
a "negative input system". These systems work the reverse of what you
might expect! They don't have the bulbs constantly grounded with power
applied only to light them - instead they constantly provide power to
the bulbs and switch the grounds to light them. I have no experience
with such cars, but I do know that the tips listed above won't work
with them! To determine if you have a negative input system use a
voltmeter to determine if power is always applied to the bulbs.