What should i do because my el camino has been sitting for 15+ years

It can only be a faulty switch. See the two spade connectors here? Remove your connector and put two wires direct to the battery. If it stops and starts as normal when you make and break the connection, it proves it is the switch that is faulty.

https://www.ebay.com/itm/For-1971-1983-Chevrolet-El-Camino-Washer-Pump-43944ZX-1975-1972-1973-1974-1976/293378902797?hash=item444ebecb0d%3Ag%3AlJEAAOSweoRd9peG&fits=Year%3A1983%7CModel%3AEl+Camino%7CMake%3AChevrolet

and the switch

https://shop.advanceautoparts.com/find/chevrolet-el-camino-wiper-and-washer-switch

..

Vehicle Information VIN: Year: Series: Chevelle Assembly Plant: Fremont, CA Serial Number: https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiln-Kd7cfSAhUo7oMKHY-uCUEQFggaMAA&url=http%3A%2F%2Fwww.autoquid.com%2Fcar-detail%2Fel-camino%2F1969-Chevrolet-El-Camino-SS-396-1969-El-Camino-SS_232257612079.html&usg=AFQjCNHsi5C3ROemGzZSaqoTCmyreD52qw&sig2=5O9CBA0vEp5PHK9brIy7eA&bvm=bv.149093890,d.amc , 69 ss

www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/El-Camino/1983-Chevrolet-El-Camino.pdf

not in the bottom of the WASHER FLUID RESERVOIR. It is mounted to the firewall. It is attached to the wiper motor.
Check out the video for blatantly obvious location.
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Wow, quite an undertaking. I had an '81 El Camino and wish I had never sold it. Originally it had a 267 c.i. V-8 with a Rochester 2bbl carb. I put in a Chevy 350 4-bolt main crate engine, Edelbrock intake and carb, and Hedmann headers (it ran like a scalded dog LOL). My advice is to look for a Haynes manual covering the '71 model year. They have complete wiring diagrams in their manuals, but the wire routing will be up to you. Good luck.

Could try just removing the valve body & cleaning it

have your starter checked out and also check you fuse that runs from you battery to your starter it sits on the fire wall

Long time sitting so the carburetor will need to be torn down and rebuilt first. The fuel tank will need to be cleaned as well. Radiator shops do that with a pressure check. Change the fuel filter and a new gas line is recomended as well. Varnish is in the lines as you may know from sitting for ten years. All rubber hoses are dry rotted too although they may look good to the eye. The radiator will need to be pressure check and the heater core too. Coolant sitting in these parts will soak the metallic parts and soften the metals causing build up to plug these cores up from age. You will be better off replacing the radiato and heater core for I know if not drained ten years back they will be no good to use

go to your local part's house and get a Haynes manual. it should have what you need.