rear defrost switch simulation

Hey guys I have a 2002 Pontiac Grand Prix and I would like to add the rear defrost to automatically come on with the remote start of my Viper 5704 alarm. I've done the research and the only way to retain the default settings on the factory climate control system is to solder directly to the board. The best way that I can see to do this is to simulate the switch on the board by using 2 wires and connecting them to a relay that when it receives and negative pulse from the viper alarm the coil will be energized and the switch will connect the 2 wires simulating somebody pushing the button. I'm not sure of how I can set a relay up to do this so I would like your help :-)

Here we go again, around the bloody houses when the answer is so simple. Find the "hot" wire to the element, trace it back to the front (BCU?) Cut then front end to 87a. Glass (rear end) to 30. 30amp fused constant to 86 and 87. Aux to 85. Diode 1N4004 between 85 and 86, band to 86.

That will retain the factory set timer for the defrost? The button on the circuit board controls an led that is on the same timer IE - push the button= led and defrost come on (button doesn't remain depressed) somewhere on the circuit board controls how long it runs and after about 10 minutes the led and defrost turn off all by them selves. I read that the only way to keep these factory settings I'd need to solder directly to the board. I took a shot at making a simple relay controlled switch using the vipers (-) output, which would look something like this. Alarm (-)= 85 12v Constant (+)= 86 Wires (soldered to circuit board)= 30---->87 This would turn on the led along with the default timer since its controlled by the circuit board. This would be a one time flick of a switch by the viper. Your example seems like a constant switch as apposed to a momentary, like a trunk release? Mine might be a little over the top because I'm soldering to the board itself but it allows the automatic rear defogger to come on and shut off with out me doing a thing because the alarm will sense the temperature and if its below 55*F it automatically switches it on. My goal is to retain as much factory function as possible.

I hope discussing this is perfectly ok I like to learn and share my ideas. I don't want to have to think about turning on the defrost when the unit itself senses temperature and will automatically turn it on. But I also don't want it running longer then is needed, so having it retain the factory timer on the board solves a few things. Its less complicated and I also don't need to worry about forgetting its on. And less running of wires. But I'm only brainstorming (my goal was to make a simple on/off switch. The coil would energize and make the 2 circuit board wires connect which would start the led and timer. Then the relay would stop and factory settings would stay on till the timer reached 0 :-)

Nothing wrong with the idea just think of the damage and $$$$ if you do something wrong that's all.

I would only screw up my hvac circuit board if something happened correct? There really cheap used on eBay. I don't see a simple on/off switch hurting components because its basically a trunk release relay that would only flick on for a second and then off. I've done circuit board soldering before and its really simple. Could you add any added protection to my idea? A fuse or something else? Or does my way seem perfectly safe? The relay is only connecting the 2 wires on the board. I'm not feeding the circuit board +12v lol. I'm just making the car think that someone pushed the switch by making the connection on the board (not feeding it power (its only a switch).

You know I've been seriously thinking about what you've said instead of jumping down your throat though you must realise your alarm thread gave me reason to have that knee jerk reaction! Any way if those boards are that cheap and you're prepared to have a go, why not! At a guess this is the internal circuit:- rear_defog.bmp You would only need a momentary from the alarm to trigger.

In fact to come on all the time, make that your 2nd NEG ACC output, orange at H/22?