For hooking up the lights on the trailer I built my own wiring system, which should at least reduce the current drawn to the trailer through the car's lighting system and (maybe) avoid messing with the alarm electronics. The only thing I draw off the car power is the running lights and a little current for switching the brake/tail light. The brake/tail lights probably draw the most current, but one could easily also add one more relay to my system to not draw power for the running lights directly from the car either, but to date I've had no alarm weirdness with the stock '05 alarm.
First I went to Autozone where I bought a little thing for converting separate brake/turn sig lights (on the elise) to unified brake/tail lights (on the trailer). I spent 2+ hours installing it, and trouble shooting it and eventually tried hooking leads directly to the battery... in an attempt to get something to light... As far as I can tell they sold me a little plastic brick with wires stuffed in the end. I've got zero evidence that any of the wires actually connect inside the molded plastic brick. Total Junk...
Perhaps I'm just unlucky, but being rather annoyed with the whole thing, I decided to do it myself (eliminating the question of whether or not shrodinger's cat caught the mice before they ate all the wires inside the little black plastic box or not)
What I implemented was something that one of my mechanics suggested. He's a short wave radio and electronics geek on the side, and had the key idea about grounding through the opposite light bulb... It was his idea, so if anything goes wrong I blame him
Try this at your own risk of course
Basically it works like this:
- I took a couple of relays from radio shack (I used the 30A automotive ones... total overkill but they had a depleted selection of relays),
- Spliced the one end of each the relay coils to the brake light wire on the elise
- Spliced the other end of each coil to one of the turn signals wires on the elise
- I wired the normally open side of the switch portion of the relay from the batery (with an inline fuse!) to the turn signal on the trailer
- All of the above was done putting the relays in a project box, and soldering the wires to the relays so that everything was protected from the soft top flopping around in my trunk.
- The running lights I just wired normally tapping off of the elise's running lights.
This works because when neither tail light nor brake are lit no current flows through the coil and the relay remains open because there is no voltage. When both tail light and turn signal are lit the relay also remains open because both ends of the coil are +12 volts and no current flows. If the brake is on one side of the coil is +12 volts and the other side is attached to the light bulb of the turn signal, which is attached to ground, so a small current flows through the coil, through the bulb and out to ground (the coil has significant resistance and won't let enough current through to light the turn signal). The same thing happens if the turn signal comes on when the brake is off only in reverse.
If you are braking and preparing to turn, then when your turn signal blinks on and off and your brake lights remain on. This means that the trailer lights are on when your turn signal is not blinking on and off when it is... So the trailer blinks in opposite phase with the brake light if you are braking and turning which looks even cooler anyway
Once I built it (about 2 hrs including trip to radio shack) I installed it in about 20 min (if you don't count time fiddling with one of the splice connectors which is unrelated).
Oh and I posted a pic of my car towing the trailer here...
and yes before you ask, I did have a strap over the tires... I just forgot to take the picture before I took it off