Minoxidil and finasteride will prevent further hair loss and thicken up what you still have. I highly recommend them.
Finasteride is a hormone based therapy which prohibits the production of DHT which strangles hair follicles, but it can effect male reproductive health so I recommend taking it typically instead of orally to prevent systemic exposure.
I have been using a combo spray for almost a year with great results.
You need to put in a double pole relay. Disconnect the horn from the original switch entirely and connect it to one of the relay poles, connect your circuit to the other.
Or just use a single relay and put the horn and your other load in parallel as long as they both run on 12V.
The horn button gets connected to the coil of the relay.
Relays don't have resistors necessarily in their coils, but the coil is designed to draw a certain amount of current at a certain voltage. In essence it is a resistor. If it were to pass the full current required to drive a car horn through the coil, it would catch fire.