Bite the bullet
Odds on its the lift pump or its wireing eather way your gona need to get access to it
Now theres two ways to do that! The first is drop the tank! Not a five minute job especially if its been untouched for 15ish years the other option is to cut a access hatch in the boot floor! (Actually under the back seat)This is alot quicker and easier if you messure carefully but it comes with draw backs!
If as said earlier the tank has been untouched for 15years the brake pipe above it (which is only accessible with the tank removed) will be in a sorry state. Its a good idea to spend the extra time and effort to remove the tank and replace this pipe.as I found to my cost about a month after cutting the boot floor to replace the pump the pipe in question bust catastrophicly rendering me at motorway speed without brakes and only by gods grace I made it to the grass inbankment in what could only be called a controlled crash! there's a thread on it somewere. Ill try find a link and you read it and make you own decisions