Anyone else suffer from intermittent sound issues from the retrofit AUX mod? If you fiddle about with the wire it will sometimes work right speaker only, then left, then it will be fine, then neither. Is it a loose connection, a faulty wire or something else?
Try 'jacking the connector block at the head unit out and in a few times Probably a 'dry joint' or high resistance contact.
You will have to face the dreaded centre vent removal to do that! good luck
If you're only getting one speaker etc, I'd suggest it's the 3.5mm jack wire you're using.
From experience, if you only half plug it in you get one speaker (or if the car think's it's only half plugged in). As they're only a couple of quid I'd change that first.
My Aux lead in the back of the head unit really isn't very secure, I'm expecting it to come lose at some point. I'd definitely check it's in firmly, and perhaps tape it in as well.