I am looking to add two more speakers to my stock Yamaha system.  Is this something I can easily do by just tapping into the existing speaker wires?  I have the feeling it can't be that simple.  I would like to keep the functionality of the volume control and being able to switch from Ipod to Garmin, but again I don't know if I can.  For you guys who have added speakers, I would appreciate learning from your experience.

You will need a 4 channel amp to power 4 speakers. To keep the stock volume control, GPS, and iPod use/control you would need the new amp to have a high power input. This means the speaker wires from the current stock Yamaha amp would run into the new amp via high power input. Then wiring to the new 4 speakers would go from the new amp to speakers. I did this originally before completely gutting the stock music system completely. Still have GPS run into new head unit via aux input and original 3.5mm cable from the GPS, but all other OEM stuff like amp and volume controls on the handlebars are gone.

I've got the alpine amp with Pyle speakers. Honestly not thrilled. There's just nothing after 65 mph. I listen to mp3s mostly and radio has a little better volume, but I wish I had gone with even more power In The amp.

The only reason I am adding speakers is my JPD lower storage boxes got destroyed in a fire at an upholstery shop. Couldn't find any replacements so I made speaker boxes. I just thought I might as well hook them up correctly.