rear defrost, alarm

I just installed a Viper 5701 alarm with remote start in my 1996 Lincoln Mark VIII. The DEI printout for the car and the wire sheet on this site don't list a wire for the rear defrost. On one of the forum's unique to the car, the best explaination I could find was this:

Without taking the switch apart and soldering wires in it that are triggered by a relay to act as the button, it isn't going to work well.

You could wire a relay directly into the power for the grids, but that would be a bigger wire to handle the current and it wouldn't be timed etc unless you can program that in with the alarm/relay. Modifying the switch with the wires is the easiest way imho, as it keeps the timing circuit and will turn off etc like normal.

Is there a better, or easier way to go about hookin up the rear defrost? How would I find the wire I need to tap? Do I need to worry about the timing circuit thing?

I'd be interested to see if there is a better way too, as I have a '94 Mark VIII, subscribing to this thread :)

Here is a link to the thread where he got the info from me, if anyone wants to see the switch etc. I can post up a wiring diagram too of the switch.

How's the rest of the install going by the way?

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Chris,

I 'think' that the switch in our cars is similar to #1 you listed. There is heavy gauge wiring coming to and from it, to the grids on the mirrors and rear window. Here's one of the pictures I posted of the switch internals below. My suggestion and the switch I have that works, is solder wires off of the internal switch (the back side of the green dots in the pic below) and use them via a relay to activate it on/off (the relay would act like the switch then.)

Is there a better way to do it? There's more pictures in that link, and I can put up the wiring digram for it if you want as well...

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Thanks for the info Chris.

Hopefully shagdrum will chime in soon again ;)

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If you do it the way Chris suggested in the above post, you'd need a 40A relay or higher. This circuit is protected by a main 40A fuse in the engine bay then goes through the defrost switch to the rear window element, it also branches off to a seperate10A fuse in the I/P fuse box (fuse 13), then to the side view mirror elements/compass module.

If you pull the connector off the defrost switch, you'll see 2 of the 5 tabs on the switch are much wider than the rest on one side (mine has the #'s 1 & 2 on it if you look close.) These are the two wires you'd use. The first one from the edge goes to the defrost grids, the second is the constant 12V input from the engine bay fuse panel, so you could use it on pin 86 as well like Chris mentioned...

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I finally got around to installing my modified switch in my car, and yes, I know it's been a year +, lol...

I used a relay to act as the momentary switch in the defrost switch housing like talked about above. I put the aux.1 wire from the remote start to 85, 12V to 86, and the switch wires from the defroster to 30 and 87.

The aux.1 wire is pulsed so it actuates the relay in place of the button you manually press on the dash for the defrost. This seems to work fine under remote start and normally, so this year I hope my defrost works when I want :)

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