Why do you suspect relay is defective?
Does the horn keep going off - seemingly by itself... like when you brake hard, or take a corner too fast, hit a bump? If so, it's more likely worn out bushings in the horn bracket in steering wheel.
Or, if it's not working at all (or only one tone), it's likely the horn(s) themselves are bad.
Bushings and horns going bad are common problems and easy DIY fixes.
You can always disconnect the horn and using a multi-meter test the wires but before we go there, let us know why you think it's the relays?
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