Idle rpm jumping when turning f.e. rear window heating

[Razor_nl]

Today I noticed something weird. It was raining and cold, typical Dutch weather. (yes, in july) My rear window was getting all foggy so I turned on the rear window heating.
Usually I do this when driving, but today it happened to be that I turned it on when standing still in front of a traffic light. I turned it on, and I felt and heard my rpm go down (to about ~750) for a few secs and then go up again. I didn't really pay attention to this since my idle isn't perfect anyway. When coming to a stop it goes down to 100rpm and then stalls or goes back up to an idle rpm of about ~900, sometimes dancing between 700~1000, sometimes stable. Not sure yet where this problem comes from, I have a feeling it's my ISC, need to measure it. The car runs just fine besides this.
Anyway, a bit later on my trip my window was clear again so I disabled the heating again. Again, I did this when standing still. I noticed my rpm went up to ~1000 for a few secs, and then went down to ~900.
So now it got me thinking... huh?!
I turned on the heating, rpm dropped a bit and went up again. Turned it off, rpm went up a bit and then down.. etc. So when I reached my destination I parked my car, let the engine run and went testing a bit.
Turning lights on/off has a similar effect but not as much change in rpm.
Turning airco on/off has a similar effect, same rpm drop/gain.
Turning radio on/off has no effect.
Turning boost controller or AFC neo on/off has no effect.
Turning blower on/off has effect but not as much, about the same as lights.
I do have a decent audio install (its my pure daily driver, not track or drag) but the amp for the subs is connected via a powercap and the music was off anyway, so I assume this isn't related to it.

I tried it also in my mom's '08 Kia Picanto () , my dad's '06 Opel Vectra and my gf's '95 Mazda MX3 (all like 100% stock) and none of them showed any change in rpm after turning any of the above on/off.

So now I was thinking... HMMM what could this be. I've read something here yesterday about the battery not being able to get charged fully causing problems like I have with the stalling. This sounds logical to me, maybe if the battery needs to power extra devices (such as the window heating) it gets a power drop and therefor rpm drops a bit because the ISC gets less power? Is there anything else that could relate these things together ?
Maybe this sounds really stupid, I'm not that much of a technician.

Of course I can go testing things but I'd like some input from you guys before I go do stuff. Like I said before, I'm not a big mechanic unfortunately

 

[luv2rallye]

The 2g has an alternator (alt) with 4 wires instead of the 1g's 2 wires. One of the extra wires (green wire - "FR signal") is an internal alt load sense line signal to the ECU (pin 41) which controls idle speed (IAC) when extra electrical loads are present on the alt. The other (white wire - "G signal") is a control signal from the ECU (pin 33) to the alt which forces the alt to output 12.5V maximum. The ECU grounds this signal when it determines it needs maximum/more hp (eg. WOT) so the alt won't use hardly any hp thereby providing more to the drivetrain. I suspect the FR signal is being used in your case to stable the idle speed.

 

[Razor_nl]

Ok I sort of understand. I'm not quite sure what an alternator is and what it does, but I'll look that up. And probably I'll know if I find the word for it in my own language
The story about the 2 extra wires does make sense since I'm using a JDM GVR4 6-bolt engine/ecu (lets say 1G) in a 2G car.

Could it be that if there is something wrong with this G-signal that it would cause the rpm drop / stalling when coming to a stop? So that the alternator gets a signal to hardly do anything so it won't tell my ISC to start working ? And can this be related to a weak battery also?

 

[luv2rallye]

I didn't know you are using a 1g ECU. The 1g ECU does not support the 2 extra alt signals. You probably have the 1g alt then with the 2 wire plug (2g alt has a 4 wire plug).

 

[Razor_nl]

Ok I'm a bit confused now sorry. Like I said I'm not a mechanic myself.

What's the conclusion you're trying to make with your last post? Are you able to tell me what part(s) is/are causing this problem or which one(s) I should check, and how?

On a sidenote, I tried a bit more with this.
Opening/closing windows doesn't give much effect, just a little rpm change. But when the windows are close and I pull the switch as if I wanna close them more, rpm drops a lot. (same goes for when the windows are fully down and I press the switch)
When I pulled both window switches up while both windows are close, rpm dropped really low. When I pulled both switches and at the same time turn on the rear window heating, the car almost stalled .

I did have all my belts replaced by the way, one of them (v-belt in dutch, dunno if that makes any sense to you) was really bad and almost split in 2. And since they were replacing it, I asked them to replace the other belts as well. Not the timing belt, that one is only 10k miles old. So it's not that a belt is slipping and not driving the dynamo enough or anything. Again, no idea if this makes any sense, I'm attempting logical thinking