It looks like they use a grommeted feed-thru w/ the relay/fuse box in the cabin. GM 80-90's trucks are like that. I only know because I stripped a few engine harnesses to salvage the knock module wiring.
Several of us took a different approach and put an underhood relay/fuse box from the junkyard (~$15). That keeps the big loads under the hood and the bulkhead connector carries only small actuating signals. You can then have relays for the lights and eliminate the large factory horn and starter relays on the firewall. Either way, it is a lot of planning and work.