My Imperial Raider collided with a GR-75 last night. We understood that we were able to take actions as normal, and to treat the touching ships as per the "rulebook page 17".

This means the ships can't fire at one another, and we played it that way. BUT is there a rule I am missing somewhere that would allow the rear of the Raider to target and fire on the GR-75?

Huge ships count as a single ship consisting of two bases, one ship token, and one or two Ship cards. The ship token is divided into two halves by the blue center line. The front base and the front half of the ship token comprise the fore section. The rear base and the rear half of the ship token comprise the aft section.

So nope, if any part of a ship is touching the huge ship then neither section can target it with an attack or be targeted by an attack from it.

"rulebook page 17" is referring to the old rulebook, in the Rules Reference the Overlapping Ships rule can be found on page 15 (that's how out-of-date the huge ship rules are ).