My modification of the glow plug wiring so that my glow plug only needs to be on for 5-6 seconds before cranking. Essentially what I did is to mount a small starter solenoid (the type that is used in garden tractors and available at auto parts stores) right at the engine so that the heavy glow plug current was now switched by the solenoid rather than thru 20 feet of small gauge wire to the engine control panel. So, the switch at the control panel turns on the solenoid, and the solenoid turns on the glow plugs.
The solenoid has a black cap that covers the two studs at the top. These studs get the thick (#10) green wires with ring lugs at the ends. Green wire on the right goes from the solenoid down to the large starter lug. The green wire on the left goes from the other solenoid stud to the glow plug (and jumpers to the second plug.) The tan coiled wire is the wire that originally went from the glow switch directly to the glow plugs...now it goes only to the small screw on the solenoid. The solenoid metal mounting bracket must be grounded to the engine.
As for the battery wiring, there's no changing the length of cable from the batteries under the nav station to the engine. Just make sure they are well protetected and of adequate guage for healthy cranking.
The solenoid doesn't have to physically be mounted on the engine, but it's bracket must be grounded so it's coil has a return path. Perhaps there is an area next to the engine for mounting space. In my case I put it on the engine to keep it close. With the starter circuit in particular, there is no fuse or breaker, same as a car. The protection, therefore, is to keep the cabling short and protected. The solenoid circuit is an extension of the starter cable, and good to keep short and protected as well. Hence, my mounting location directly on top.
In my case putting the solenoid on top was also a way to minimize additional cabling between engine and the rest of the boat (or is it a yacht?) I want to put sound insulation in someday soon, and making the area around the engine as free as possible of "stuff" would be helpful when I install it. I suspect I'm going to have to make some major modifications in the process because the engine is already squeezed in that space.