Peugeot boxer autosleeper

Hi, as you can see, I,m new to site and really need help. Bought 1995 boxer symphony, I think and only went out last week in it. On motorway driving fine, but when I turned off for lakes (all downhill) it started backfiring and spluttering and when I tried to go up a hill, no chance. Cut a long story short put in 4 new spark plugs and 2 that were removed were full of water rusty and black, anyway absolutely fine and driving great. Looked to see where water could be getting in and found that black trim at bottom of windscreen has been siliconed and when I pressed washers the water is running down onto aforementioned plugs. We also went through a bad storm coming home yesterday and stopped at services, and water pouring in to engine. Any suggestions would really be helpful, thanks Eric