What ohm resistor to go from speaker to headphone

I'm sure this is easy for you guys. Maybe you could give me a starting point.

I put a jack into a Garmin GPS that cuts off the speaker and routes the signal to a pair of ear buds. I have to turn the volume from 80% to 10%. Can someone guess at what value I might want to insert a resistor in line to better match the earbuds to the speaker. I'm afraid the earbuds are too low in resistance and might burn out the amp.

Thanks,

Floyd