C3 I 1.6 (TU5) battery light on when car is off, ign on, starts flashing

[h_ajitani]

I described almost everything about the problem is occuring to this car in my shop, but it's this, when the car is off, the battery light stays on, permanently, when i turn the igntion on it starts flashing and when i turn on the car, it goes out.
Alternator is charging perfectly (13.8~14.3), recently the voltage regulator was replaced because the car died when running and this main problem was solved, had to charge the battery a little.

Sometimes the lights on the car wont turn off and wont lock the doors but when running, everything is perfect. Other shop said the BSM was defective but i didnt find anything and didn't replace this, just cleaned the terminals and every fuse and that's it.

This car also has an aftermarket multimedia which i think it's plugged directly to the ignition, nothing's connected to the BSI via some fuse or something similar and that's it, my point is if this aftermarket radio can cause something like that.

[ozvtr]

Depending on the year, there are two types of communication "BUS" types for the C3. CAN (Controller Area Network) and VAN (Vehicle Area Network). This "BUS" is the way the control units communicate with each other.
You haven't said what year the C3 is.
If the multimedia center is connected to the "BUS" incorrectly, then yes, you can get random things happen.
The multimedia center would be connected to the BUS to allow things like the radio controls on the end of the wiper stalk to control the volume and radio tuning.
I would suggest removing the multimedia center completely and see if that solves your problem.
If it does solve your problem, then I would say the multimedia center has been installed incorrectly and just needs to be installed correctly.

[h_ajitani]

It's a 2010 C3, I forgot to mention, facelift version.

I removed the multimedia but the problem still persists, aside from that I noticed, now that I removed it, that there's a connection from behind the dash cover which I didn't digged in to see where it's going, what I see is that a sound cable connect to another BSI terminal around the steering column, either way, would this be related to this problem? Even with the multimedia disconnected?

[h_ajitani]

I found a (aftermarket and of a terrible brand) module to close the windows and a terrible installation, and when I do poke around it, sometimes the battery light goes off with the car turned off, but as soon as I turn on the key, the problem goes back, so I'm assuming the problem really is there, in the installation.

All of the wires go way back to the rear buttons and as soon as I plugged the multimedia back I heard strange sounds inside it. If the client approve its removal and if the problem is solved, I'll post here but I doubt, he will probably rather live with the problem than remove the (stupid) screen.