Disco 2 Damp in dashboard

I have a Discovery 2 TD5 1999 that my dad has owned for many years and has been parked up since 2015. This year I have finally come around to getting him back running so I thought. Mechanically every is as it should be and I’ve re done anything that wasn’t right.

However he still isn’t running and there is damp in the dash more specifically under the steering so I took the fusebox out and all of the fuses were corroded so I air dried it and replaced all the fuses. He is now really cranking over and wanting to start the most he has been since the start but at the same time the immobiliser light is dimming glowing stopping him from starting in my opinion , there’s two sunroofs one above front seats and one above back so my next guess is water it’s leaking down into the dash causing a short and for the disco to not start. My thoughts was to get a tarp to cover the roof and buy a dehumidifier to try and expel the damn hopefully drying out any electrics

Any tips are appreciated

There is probably corrosion within the fusebox between the pcbs too where you dont have any access and no dehumidifier would fix that cos the circuits can be also affected, better buy a known good one removed from apre-facelift and swap it, it's almost plug and play, disconnect the battery first then after the swap and battery reconnection turn ignition on 2 and let is so 5 minutes, that's all...if this doesnt fix it then the BCU might be affected too by moisture and that one needs complex programming