I need a new horn for my ef Fairmont. Would AU and BA horns fit ef's? Im guessing they all just connect much the same way to the 12 V? am i right?

Yeah, will work. If you have a single-tone horn then you will need a wiring kit to have dual tone horns, but you have a fairmont, so i'm assuming your horn is dual-tone anyway (hi and low) so any of them will connect up.

Same with my Au futura, single tone horn... want a hi-low dual tone like the fairmont/fairlane/BA xr's have but ford want $55 each horn and $20 for wiring kit... maybe i'll go to a wreckers...