The solenoid for the aft head is affixed to the engine room bulkhead that runs fore-and-aft and is on the starboard side of the engine room (closest to the head). As you are looking into the engine room from the main doorway (opposite the nav station), it is just next to, and behind, the forward vertical steel post that comprises the battery support structure. It's about even with the top of the compartment. If you follow the run of the 1" hose that runs up to the top of the engine compartment, and then back down, you'll see the solenoid at the top. But as you're looking into the compartment, it "hides" behind the upper battery and battery support structure, between that bulkhead and the upper battery.