I didn't use a relay in my positive ground 65 +4. Knowing which way yourMorgan is grounded is vital knowledge whether or not you use a relay.
As suggested the relay is ideally necessary and
will work with a postive earth vehicle. Just ensure the positive connection on horn goes to vehicle chassis and the relay coil terminal 86 has a the OE horn switch connection which completes path to chassis earth when pressed (separate switch could be used wired similarly to provide switched earth). Terminals 30 and 85 can be connected together at relay and fed from fused feed of battery negative. Finally relay terminal 87 needs connection to negative terminal on horn. Ignore terminal 87a if five terminal relay.
This will work for any isolated horn but beware if any with single connection and earth return through metal mount casing on postive earth vehicles.
