Q:

Correct Relay Splicing Methodology (Backup Camera & Reverse Wire)?

Some of you may recall my previous thread where I asked what the best way to splice a webcam into the reverse wire of a car. While these questions are similar in topic, this one focuses in on the specific use of the relay, and the problem I have encountered while wiring it.
Basically, I spliced a 12v relay with a maximum current of "30/40A" into the reverse wire of my 2005 Jeep Liberty. What this means is that I cut the wire in half and connected each loose end to its correct terminal on the relay (I believe they are the 86 and 85 terminals, if those are standard labels). I turned on the car and put it in reverse, and the relay switched. But, when I went to the back, the reverse lights where not on. Here's a picture to model this:

http://imgur.com/Dj0Ehoy
Shouldn't the interior of the relay on the coil side just be a continuation of the circuit? Why would it interrupt the power so as to prevent the lights from turning on?
Then I decided to take out the relay and just connect the loose wires with solder to make sure that it wasn't my soldering causing the problem. This indeed turned the lights on - not a solder problem.
Following that, I soldered the two loose reverse wires together like the last step, and then just soldered two new 19AWG wires (the same if not thicker than the reverse wire) to the reverse wire, then connected those to terminals 86 and 85 on the relay. This turned the lights on, but the relay did not switch, as modeled here:

http://imgur.com/nPL2guF
So my question is: Is there some electrical step that I am overlooking, or is there something wrong with my methodology here? What would cause this split in results between the two tests?
Thank you for your time.

A:

You can't do that either way you tried.  You need power +86 from the reverse wire, and a suitable ground path +85 for the other side of the coil.  You'll have to add wire and find bare metal or another ground wire somewhere.
The coil provides a resistance, such that very little current flows through it.  This is why the reverse lights didn't come on in the first case, but the reverse light bulb filaments provided enough of a ground path to close the relay contacts.
Relay Basics

A:

Neither of those diagrams would work the way you want them to.
In diagram one power never reaches the lights as up to pin 86 is grounded.
In diagram two power reaches the lights, but pin 86 is never grounded so the coil never closes.
Try this.

85 and 86 are dependent on how the reverse light circuit works. If the circuit is normally grounded this diagram is fine. If it's normally powered 85 and 86 would be reversed.
When you tap into the reverse circuit you can either use a tap

or strip back a portion of wire and solder the wire in place.

