I am installing a anti theft starter shutoff relay in my 73 challenger. This relay breaks the ignition lead from the steering column to the starter relay. 1 wire to starter side, 1 wire to ignition switch side, 1 wire to ground, and 1 wire to negative side of an on/off switch.
I have a rear defog switch and planned to use it for the on/off switch, only problem is that I don't have a wire connection in the new harness, my thoughts were to run 1 wire from the switch to ground and 1 wire to the relay that called for a negative lead. Will this work?
This relay will activate when the ignition is turned off and will not put power back to the starter solenoid until you turn the switch back on, once the engine is started you can turn the switch back to the off position.

I think you could make this a bit more simplistic.

take the neutral saftey wire from the transmission. Wire it to one side of the defrost switch.
take a new lead and run that to the starter relay.

if the switch is off, no ground to the relay and turning the key does nothing.

if the switch is on, AND the car is in neutral or park, it will start normally.

no relay needed, and doesn't matter if you forget to turn it off when you are done. and works as you described you wanted it to work.
no new leads needed in the original harness, and no cutting.

Anybody that wants to steal your car knows how to jump the coil and spin the starter by shorting the solenoid with a screwdriver. I prefer to disable my spark not my starter.

They don't make car thieves like they used to. I hear less and less stories of screwdrivers jammed in the column. My car does not start like a normal car- the starter is on a hidden momentary button and the key and killswitch control the spark. If you turn my key to crank it acts like it's dead

I put a relay into the coil power wire. In the past, I used the lighter as an ignition ground, push it in, and it grounds the coil. If the engine turns over, thieves will try for a bit, and then maybe give up. Hidden switches are fine, but you could be seen reaching. How about an extra dimmer switch on the floor? Heater switch in cold areas might not be the best, but a good idea.

You could just run a wire right from the coil - to a switch, with the other end going to ground. Flick the switch and the coil gets grounded out. She'll crank and crank and crank but won't fire. The casual car thief will give up at this point. One determined to get your restored mopar will be there with a truck, trailer, winch, tow truck, etc.

I wire a single pole double throw switch on the ign wire & the start wire, when you throw it to one side the horn blows when the ign should be powered & the starter is disabled as well
I've done this for years & me nor any of my customers that have them have lost a car yet.

OK here is my take.

If I were gonna steal an old Mopar I would run a jumper from the battery to the ballast, then crank with a screwdriver on the starter relay. Switches that break circuits will have no effect.

My opinion is to slip into the wiring harness and tap a ground wire into the coil/ECU wiring. Something you can't see because it's taped up real good and use your relay or switch to ground that wire. A dead short will stop it cold and nothing short of some creative handywork by somebody who knows what they are doing will get around it quickly. I suggest you put a resister in the short circuit to prevent a toasted wire. Most thieves want quick and easy or they don't want it at all.

One creative idea used by one guy was to turn the cigarette lighter into a switch. Nobody would think of that one.

My

You can use the switch & with the horn going off they jump & run the time for creativeness gets really short
My same wiring can be used with relays without dead shorting anything
Most of the cars & trucks I wire like this are for people who live in the hood or just outside it & the success rate is really good & catching the creeps rate is good too one guy managed to shoot one with birdshot in the backside

What you would do is cool but real car thieves don't raise the hood
The real guys unplug the ignition switch & plug another in in the older cars if they don't pin them with a QuickPick in the back of a truck The seatbelt becomes a tool to hold the steering wheel

A person could experiment with ground wires & resistors on the newer stuff that's surge & ground sensitive
Sounds like a possible costly experiment

An awesome trick is to use a magnetic reed switch to trigger a relay:

http://autospeed.com.au/cms/article.html?&A=107975

This system allows a lot of creativity in placement and if you do your wiring right, it's impossible to detect for all but the most dedicated thief!

On newer cars that have more complicated dashboards it's even easier to hide everything.

I think you'll be ahead if you look for places to hide a lo-jack GPS sensor. As said, a REAL car thief will have an enclosed trailer and a winch.

Most cars stolen involving an enclosed trailer they are already in the trailer to start with

Did that on my 69 GM 3/4T work truck. Put in an electric fuel tank selector valve, and capped off on side. When power hit the switch, the valve was open, no power, no fuel. Small LED under the dash for power on, switched by an extra dimmer switch on the floor above the stock one. I could leave the pub, and make it two blocks before stalling. Seemed simple at the time. The lighter thing came about in my 64 Ford work truck that I installed a 6 cyl in-dash tach in.. Pondering on it one day, and spliced a wire from the tach feed to the lighter, worked great. This all came about after my 74 Barracuda was stolen.

do you have a diagram of how to do this? seems like a great idea!

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What kind of vehicle do you want to put it on?
I've wired them to trigger headlights & horn
Why let them get around the corner where the trailer or QuickPick is awaiting
How much damage can be done with 2 blocks of gas whether it be a thief or vandal?
Some of my customers use a section of frame chain covered with bicycle inner tube & a circle lock through the wheels when they go out of town
When looped through the suspension they can't steal the wheels
The bad boys have a hard time opening most of my customers cars trunks & hoods too
It takes some solid effort to open them when I'm done

Most of the ones I do have an alarm to keep the wannabe thief at bay kinda like a warning shot