Fuses can be anywhere. I have no idea where the 18” rule is from.
The question of distance of a fuse from the power distribution point is around safety.
If there is an uninterrupted run of wire from the front of the car to the back of the car the fuse could be at either end if the wire is protected and unlikely to be damaged by something. The wire running from your battery back to the starter is an example of this and it has no fuse at all. It is the largest gauge wire in the car and can carry the full amperage of it from one end to the other of the car. Is this unsafe, yes in theory but in reality not really due to it being restrained and in a compartment which protects it.
If there is a high likelihood of damage to the wire between where the wire is terminated to a power source to the point of use, a fuse should be as close to the power source as possible. The front trunk is a fairly safe space, particularly if you put the wire in some casing and securely attach it to structure periodically.
The things which damage wires are events where the wire can get pulled out of where the power is being used, can be chafed by other objects, can be crushed by other objects, pierced by other objects or cut by some other object allowing the wire to go directly to ground. The trunk is a place where some of these things can happen, more so on the floor or low on the trunk wall, less so near the top of the trunk and along the firewall where most things are unlikely to be placed. However protection and support of any wire is key to both safety and reliability, we all like the sparks to be where we intend them
If properly protected a wire can be run from the battery over to the other side of the trunk and have fuses over there or multiple wires can be run across the car and fused nearby the battery directly to the points of use be it relays for the headlights, fog lights or wipers.
Is there a bundle which you can unwrap, add your new wire to the bundle and rewrap it? As that bundle has run successfully and undisturbed for nearly 50 years I suspect it would be safe to use that path. If one doesn’t exist then running a new bundle which includes any control wires along with power to the new point of use is entirely reasonable.
Anyway, for the headlight relays which will be chewing up 100 watts of power I would run a new power wire for those relays and a separate one to the two relays needed for the wipers, I would run them across the trunk in a wrapped bundle with periodic fixing to the body and I would place it either just below the level the targa top sits or further down where the vertical firewall meets the angle coming up from the floor.
I would be tempted to fuse them near the battery in a small new fusebox with a couple of extra open fuses for anything else you may add. Get one with LEDs that show continuity through the fuse so you can tell at a glance that things are OK.
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