Aft rinse off shower 2000 390
I have the same model and year, and had the same situation.
It was a challenge to locate that supply line (its a snap fit assembly you can buy at any reputable marine store) that ran from water pump aft towards stern behind water tank. What the challenge was getting your hands in with very limited spacing to make the connections.
I planned on 60 mins job, and it took 3+ hours using a straightened coat hanger to act as a "mouse line" to get new supply line routed from pump to aft location of shower head. I replaced the handheld shower head with a new scanvik, and new supply line as all were 17+ years old. In the run of the supply line, I installed a "petcock" style shut off valve where it was easily gotten to so I could shut off the aft shower completely if I wanted to. Lots of cussing on my back, but got it done.
With this new enthusiasm, I attacked the bow shower/wash off assembly. Replace the handheld shower, but even after removing the stateroom bulkhead mirror, could not reach the nuts/joint to replace the hose without cutting another access hole. So now have a new wash down handle, with a 17+ year old hose...LOL.