Some Questions about 96 Jeep Cherokee

Hey, I have a couple of questions: First, I have problem with my speedometer, tachometer and gas gauge when it rains. They jump around. and sometimes don't even move when I am driving. They all move in unison so i think they are all on the same circuit or something. I asked a mechanic and he sid it might either be a fuse, relay, wiring, or a grounding problem. Anyone have any experience with this, or know how to fix it?
Second, I have noticed that when I turn my defrost on, the blower fan makes a loud noise when it switches on, runs and then switches off. i have read a couple of the responses and some recommended a new fan belt? Will this make it more quiet?
Third, is there anyway to turn off the Add Washer Fluid light? Ever when the reservoir is full it blinks.
I am new to this and not very handy with car/truck work, so please explain things in basic terms so I can try and figure it out. thanks - John

(1) What sized speakers are in the 96 Jeep Cherokee?

(2) What type of oil and what size oil filter does it take?

And sorry but I'm not sure about your questions.
However I have a similar question regarding the gas gauge :

(3) My gas gauge isn't very consistant, by that I mean, when it gets to about the "one fifth remaining" mark, it just sits still then drops down (causing light to appear) then goes back up to about q quarter and back and forth till I gas up. What's the deal with this?