rear defogger

I want to use relay to turn on my rear defog with my alarm, i tested the wire for the rear defog when defogger off its show +12v and when it on then its show - ( nag). Is there a way to wire the relay to normally output + (pos) and when the alarm pulse -(nag) signal then relay switch the output to -(nag)

It appears you have a NEG (-) switching line there. Try pulsing that wire to ground with the engine running, it will either be a momentary (pulse) or a constant. If so you won't need a relay.
It might help if you posted vehicle make, model and year.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Its 2009 toyota camry, i have thought about that but the alarm only supply -200mv to the wire. I dont think it will be enough to turn on the rear defog, i have thought about using relay to turn weak - ( nag) to stronger -(nag). Question is when the coin not energize what does the out from the relay supply?

vietxtlife wrote:

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Its 2009 toyota camry, i have thought about that but the alarm only supply -200mv to the wire. I dont think it will be enough to turn on the rear defog, i have thought about using relay to turn weak - ( nag) to stronger -(nag). Question is when the coin not energize what does the out from the relay supply?

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i meant -200mA

Cut the wire and meter it.
You should be OK if it's a thin wire, it's already triggering a relay.
Simple test, do you have a test light, NOT LED. Then run engine, one end of test light to ground, probe end to wire. If that triggers the defrost, then so will the alarm's output.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I have the powerprobe and its can supply + or -, so i try to supply the suspected wire with - and i can hear the defogger relay click. So now do i need to cut the rear defog wire and tide it to my alarm output, or just connect to it without cutting. Still the alarm output a weak - dont know if that affect anything?

What output are you using from an alarm? The rear defogger can only be used with the vehicle running as it draws a lot of current. Is the system designed to give a latched or timed negative output? Some remote starters are designed for this function, but alarms are not. That vehicle needs a latched ground on the Green trigger wire.

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sparky