The metal immediate around the round rubber plug at the passenger-top side of firewall for electrical wire to go inside the cab is corroded.(bottom side) So whenever it's raining, water runs down the windsheild/cow...thing & somehow seeps to the inside of the cab just from the top of the wheelwheel.

I tried to plate it with metal but welding gun is to big to reach & it's way to close to the wiring harness. I use ducktape temporilary but water is still going inside, making my floor wet everytime.

Can anyone think of anything that i can apply in liquid paste form that let me shape it as i wish & hardens to block the hole? The gap is around 2"x1".

I'll try to post a picture as i figure it's pretty hard to understand what i described above...

You might try some epoxy putty.

I'd use fiberglass mat & resin. If it isn't too big of a gap, tiger hair on it's own might work.

I'm not sure how big it is, but I've used RTV sealant to plug a bunch of holes that I have to run wires and it works perfectly fine, if the area is small. There's also any number of different types of apoxy, JB Weld, etc that work wonders at any auto parts or hardware stores.

If you had to cover a large area, I'd probably cut out a piece of metal, apoxy or JB Weld it on there, then RTV seal around it.

cut out the rusted part.
remove paint as needed and then fibreglass/glue/weld/epoxy/seam sealer or otherwise attach the new piece of metal into place.
use seam sealer to seal it up
repaint

look here for an example. in this case he is tubbing wheel wells for more clearance. but the idea is similar.