how can i fix my 96 escort mexico screen washer?

my water resorvior in my escort mexico,which is 10 yrs old,is blocked, i think,the pump is working fine,when i go to clean the window i can hear it trying to pump water,but nothing comes out,i think the tank is full of gunk,ive had high air pressure pumped down the pipes,that was 6 month ago,now its all blocked again,do i need to take out the plastic inner wheel arch to gain access to the tank to take it out and clean the whole thing,and,is it an easy job,any help would be good cheers

graeme
March 2007