Horn = High Beams

[Mitz]

Go where what's cheaper and easier to install: a relay or a 10-Amp diode.

Maybe you can find the diode at any Digi-Key or RadioShack store, the relay may be easier to find though.

[daleWV]

Search for a 1N4004 diode on Amazon. This is a 1 Amp diode and is plenty for this application. If you or a friend has a crimping tool just crimp insulated butt splices onto the diode and your 18 awg wires. You can get a crimping tool and butt splices for cheap at Walmart too.

[foama]

Yes it does. That is quite simple. Where can I find me a diode that can be terminated to ~18 awg wire?

Just ask you local radio&TV technician for a 10 Amp diode. This diode must be able to power the full current of the high beams. A 1N4xxx is far too weak. Usually there is a ring printed around one end, which should go the high beam side. If you put it the wrong way around, the horn would sound when the high beams are put on. Also, the fuse for the horn will need to be updated from 5A to 15A to master the higher current going through it, but then, the wiring in this car is thin. Will it take the extra current?

The easy way would be to connect the diode between the relay coils, so both relays come on simultaneaously, but I would certainly not do that because I have no idea what it may do upstream in circuitry, and those modules are expensive to replace.