Skezza

It's only held in the connecting piece by gravity really. Pain in the arse to get back into place. Not actually that useful on CL cars tbh.

Does it actually run?

Does the audible alarm when locking thing actually work? Never seen it work properly in practice.

Oem speaker?

Not what the op asked though.

I like this.

Purge in the tank does nothing really. We both know that. Did you not do the full setup like I've said before. That includes running your own independent fuel source from a pop bottle or something?

Have they thrown in an aftermarket wiring loom? Ironically enough, the loom itself is aftermarket but the sockets are all ISO and would probably still work with the OEM radio. Worth a check.

Anti-Hijack with the button on the door doesn't need coding in. Automatic anti-hijack does. Takes 5 mins to code in no problem. Automatic window rolling up is not so easy and I very much doubt can be coded in. Some aftermarket kits have it included.

How often does it do this? It points to a voltage sag. Do both window switches do it? What about headlights? Being a GTi somewhat clouds the game a little because the ballasts will provide some capacitance ( @mattarosa? Am I right?) and as such a dip in brightness might not be seen. Brake or tail lights might be a good one if someone is stood behind you. I am still not convinced by your alternator.... or the earth. You said the battery has been swapped out so I'm assuming this is good. How sure are you of the voltages you're measuring? How did you go about taking the measurements? I'm also not convinced you haven't got a ropey earth somewhere.

The story continues....

On a 1.0???? You're taking the mick aren't you .... We both know it's not just that easy, let's be honest. Where's the turbo manifold coming from mate?

Just try jump starting it. Some of these electrical gremlins scream worn alternator + failing battery.... at least to me. Might be wrong, but can't hurt.

I don't know why..... But I'd be interested to see what happened if you tried jump starting it off another car with a good battery.