you shouldn't need to cut the wire, it has a spare unused connector on the end, you can connect straight to that.
Cheap radios can fail closed circuit and thus draw stacks of current.
A knackered alternator will slowly destroy a battery and vise versa. Pop down to Kwik-Fit or the like and get a free battery check. They have a tool, find out what the actual readings are and bring them back, DO NOT buy a battery from them, no matter how insistent they are!
Sounds like a combo of dodgy alternator and dodgy battery. Best bet is simply to buy a trickle charger and connect it over night, you can get them that fit into the fag socket. Works for me and has done for quite some time now.
Rewiring radio power feed
I have retested the alternator and its giving over 14v even with headlights on. The battery is showing about 12.5v. A bloke from a car supplies and radio bits shop explained to me that older cars like the BX do not have an ignition switched radio feed, and from what I already gathered, this radio expects one or it will run switched on whether the face panel is attached or not. The man sold me a relay circuit which he said to connect to the + on the fan control panel for the dial lights as they come on when the key is turned. I did try and find this green wire, but the only green wire I could find was behind the radio cavity and was plugged into something already.
After removing the fan control board (again), soldering the relay wire to it and connecting everything up, the radio now only recieves memory feed unless the key is turned. Hopefully this should ensure no battery draining goes on overnight.
Sadly, the unshielded negative wire I noticed the first time I got behind the radio panel proved to me that even though it wasnt a + feed it could still cause havok after it found its way to the + pin on the relay while I was trying to stuff the radio back in. One blown fuse later...
Anyway, I'll be testing the battery periodically until i'm sure all is fine.
After removing the fan control board (again), soldering the relay wire to it and connecting everything up, the radio now only recieves memory feed unless the key is turned. Hopefully this should ensure no battery draining goes on overnight.
Sadly, the unshielded negative wire I noticed the first time I got behind the radio panel proved to me that even though it wasnt a + feed it could still cause havok after it found its way to the + pin on the relay while I was trying to stuff the radio back in. One blown fuse later...
Anyway, I'll be testing the battery periodically until i'm sure all is fine.
1992 BX TXD Turbo