So i have a S-type jaguar 2000 3.0 and one day everything electrical on half of the car stopped working back windows , trunk , locks , and fuel pump , i connected the fuel pump directly to a fuse so when i turn it on it starts i took it to a electrician and he said to purchase a new Rear Electronic Control Module its about 300 on ebay and he said its real easy to put into the car he showed me where its at im just wondering is this really what will fix this problem and maybe some pictures thanks in advancee

 

My amateur guess based on experience here... You are getting water in the trunk...
The 2000 to 2004 body is prone to leak into the trunk.. And the built in drains are known to clog causing water to sit. Also, a bunch of relays and fuses are also in the trunk..So, not good when water gets in.
I would search here for ways to test for leaks...Just one hint..Don't dismiss the "lock yourself in the trunk" method as a joke..It is one of the best ways to find a leak and fix it. As for the module...He is probably correct. I don't think the fuel pump is controlled by that module, but it does have a relay that is in the trunk, I think.
Also, I would fix any leaks first or else you risk damaging anything you replaced the next time it leaks...

 

I looked in the trunk and I dont see any leaks in the trunk. A auto electricion came and said i needed a rear electronic control module. Anybody know where i can buy one for cheap online in case thats not the issue i dont want to waste alot of money? thanks in advance

 

I would not dismiss anything Jeff is telling you to check. I am attaching a oneline for a 2000 3.0 S-Type for you to look at. If you do dismiss Jeff’s suggestion for now then look at figure 1.1 you will see the main distribution of power to the entire car. From the battery it goes into 3 directions you need to see what distribution box is not powered. You can also identify what panel feeds the items that are not working. If all are powered. If this is not a problem you need to take Jeff’s suggestion seriously. Just do not get locked in the trunk! I hope the oneline helps.

http://www.gusglikas.com/images/Auto...01999.25en.pdf