I know a little about a lot of things. When it comes to audio, I know nothing.
I have an 07 Street Glide with stock head unit, Arc Audio 125.2 amp and 6.5" Hertz speakers. When I bought the upgrade from Audio Werks of Tewksbury, MA at Laconia Bike Week a couple years ago, I told them I may want to do rear speakers in the future. They said no problem, we'll run some wires back there for you. Now I'm hearing that the 125.2 was made to run only 2 speakers and if I want rear speakers I need another amp for them or an amp that was made to run 4 speakers. I have more questions than answers and am extremely vulnerable to being mislead because I just don't know anything about this subject.

 

As long as you add another set of 4 ohm speakers, you can use the Arc 125.2.

That amp is stable at a 2ohm load !

Right now you are running 70 watts per speaker. When you add 2 more speakers, you'll be running 62.5 watts per speaker, so no 'real' loss of power , to double you speaker output !!

 

This will work, but you will not be able to fade between them.

 

Not to jump in and steel this thread, but will you explain the hole 2ohm 4ohm thing?

 

Do you mean left and right or front and rear?

 

Do you mean fade left and right or front and rear?

 

he means front to rear. left to right will still work.

4 ohm to 2 ohm explanation: your speakers now are 4 ohm (front).
if you connect another 4 ohm speaker parallel to the existing (meaning add plus and minus wire to the amp the same way as the current speaker is connected and run to your new speaker), the total resistance becomes 2ohm.

formula (multiple ways to calculate):
Rtotal = (R1 x R2)/(R1 + R2)
Rtotal = (4 x 4)/(4+4) = 16/8 = 2

hope this helps.

When putting speakers in series, you simply add the Resistance values together.