I know this forum is mostly for tuning, but thought i'd add my own project just for fun.
Basically im tired of having an stone cold car in the morning when its -20c outside. So i've made an arudino based remote start for my mk4 Golf. You WILL have to have an immo defeated ECU or trick the immo so the car wil start without the key in the ingition. Its also an good idea to fit an neutral switch 
It uses an SIM800L GSM module to recieve SMS, and then does whatever the text tells it to. It's got 4 functions at the moment; start, stop, status and change time.
The code has everything explained in it, so just look there. I'll try to make some PCB files so its easier to understand the wiring, but its not complicated at all.
 
https://github.com/SivertGullbergHansen/GolfMk4RemoteStart

I've been thinking of adding CAN support to it, so that i can read/send CAN messages. But have not come around to that yet
If you've got any ideas on how to improve it, please let me know

The correct way is to fit a preheater such as Webasto or Binar.
Much more fuel efficient and not illegal.

It is illegal to idle an engine next to a house for longer than 2 minutes, at least in the EU.

It's also super shit for the engine to warm it up by idling. Basically worst thing you can do.
Because everything is cold and stays cold for a very long time, as the engine produces very little heat while idling.
Best way is to drive it gently below 2500 rpm.

Webasto is quite expensive, but Binar is actually not expensive at all.
We have fitted it to Mk4 Golf also, no problems.

E.g. my company sells Binar 5S for 695 EUR and this includes almost everything you need.
Optional GSM module is 135 EUR, the base model has already a small control panel with a timer you can set from the car, so the GSM module is not even strictly mandatory.

P.S. You won't destroy engine by idling cold, but the oil and more importantly piston rings and bores will degrade quicker than just starting it and beginning to drive. This is because of washing the bores due to poor atomization of the fuel with a cold engine.
Especially if you start it at -20C and just let it run. Will also take forever to get any heat going in the cabin vs a proper solution where it is heating the cabin only and 10 minutes is enough to have 25C inside.
There is a reason why this stuff is not very popular. Except for very cold places where it is -40 or even -50C, there they have auto-starters when the coolant temperature drops below a certain point it starts the engine, as otherwise it is highly likely it will not be possible to start at all.