When doing the 2 part engineer quest, try to avoid saving and exiting the game because that has a chance to de-spawn the target party icon that helps you to find them, if this is the case, you have to enable the cheat options and type in;

• left alt + tilde
  config.cheat_mode 1
  tor.whereisgoswin
  config.cheat_mode 0
  (Thanks to @Sly )