why does Fujimori do so well on the north coast?
I believe Trujillo, Chiclayo, etc. are the wealthier parts of Peru. Race also plays a part, of course; that area is mostly European in ancestry, with few people of native ancestry.
No, wealth is concentrated in Lima and along that general area. The northern coast is largely non-native and more or less working-class (sugar plantations) which were Aprista strongholds, especially Trujillo which is where Haya de la Torre was from.
That being said, if you look at the results in the wealthiest districts of the country, you'll see how Keiko trounced Humala there.