Megane 03?

Hi
I have a megane and some days it can be great and others it can be murder, there's days when it keeps cutting out at every corner and junction for no reason. had it in the garage and they say there was water in the fuse box the one rear the battery, Yes, there's two the other is in your glove box. another problem I had was when I braked the rear indicator came on with the brake light on the passenger side, had to buy new light unit from dealer £40 connections on lamp connector was rusted so water was getting some how. water runs down the body work under the tail gate and there is no seal to stop it getting in behind the light and causing the problem. I got some of that sticky back sponge and put it around the edge of the light on the body work hopfully it will stop the water getting in. Seams to me that renault dont put any water seals on there cars, so get some clear silicon and go round your car and seal up all joins and gaps you can find its amazing were the water gets in and it always ends up on something that can cause a electrical fault. So if any one can tell me why my car keeps stopping for no reason please tell me. some one said it could be because I dont use the expensive petrol, who knows. not me.