Connecting 2 add on / extra speakers

There are no speakers in the back doors of my car, just in the front doors and rear deck (2002 Ford Taurus). I currently have the components in my front doors and the speakers in my rear deck connected to an Alpine 4 channel amp. I want to intall speakers in the back doors. What I can't figure out is how I would connect these to an amplifier (I am more than likely going to buy another amp for the speakers I will be installing in the back doors. I am clueless as to how I would connect the 2 back door speakers to an amp considering The RCA outputs on the headunit are all being used. Can i just use the 4 channel amp I have and run additional speaker wire from 2 of the channels to the back door speakers? This would mean there would be two pairs of speaker wires on 2 channels of the amp. Would doing this decrease the amount of power to each speaker, or can I run multiple speakers off of one channel on the amp without sacrificing power output or causing damage or strain to the system? Ifcanyone can help guide me in the right direction it would be much appreciated. Thanks

At the risk of sounding like an adult diaper, it depends. What is the goal? More bass, fill in a void like midrange, or louder? Assuming the speakers are all 4 ohm impedance, connecting them in parallel will result in about 2 ohms nominal impedance. They will be parallel if you connect the "+" of each speaker to the "+" of the amp, and "-" of each speaker to the "-" of the amp. https://www.the12volt.com/caraudio/boxcalcs.asp I use the words about and nominal, because impedance varies in frequency, and manufacturers like to spec their impedance in nice round numbers, 4 ohm, 8 ohm, etc. If the speakers are identical, the result will be a nominal impedance that is as close to 2 ohms as their spec is accurate. If they are two different types, YMMV. You need to look up the specs on the amp and see what the minimum impedance is; going lower is dangerous for the amp. As impedance of the load, the speakers, drops, the power produced by the amp increases proportionally. So going from 4 ohms, to 2 ohms, doubles the amp's power output, also doubling the current flow and heat, so it "works" twice as hard. If you can't find the specs on the amp, post the model number, and hopefully someone will find those for you. I am not a pro, more of a hack. In my opinion, your best bet is to get a 2nd amp, then just get a pair of quality, shielded RCA "Y" cables and use those to split the rear channels RCA to go to each amp. If your amp can handle a 2 ohm load, it should be OK, but if the speakers are different, you could get some odd results. Semiconductors die from either over heat, over voltage, or after 20 + years, old age. So less heat is good in electronic devices, especially power devices like amps. If you don't play loud, less concern. 4 channel amps give the most bang for the buck, and two extra channels could come in handy for later if you decide to add on another pair of speakers, or a small sub, if the amp has the power for that. "Y" example: http://www.mcmelectronics.com/product/DISTRIBUTED-BY-MCM-JR8058-/24-770 Don't buy that one, it's a cheapie, just a visual example. You want a shielded cable. Gold plated is a plus, but I'm not convinced it's better than sliced bread... silver/chrome has worked well for years. Good luck with your quest for sound!Build a man a fire, he keeps warm for a day. Set a man on fire he stays warm for life!

Forgot to mention, if you get a second amp, and one or the other has RCA pass-thru connectors, normally labeled "OUT" or "OUTPUT" or something, you don't need a "Y" just a straight RCA cable from one amps RCA out's to the other RCA in's.Build a man a fire, he keeps warm for a day. Set a man on fire he stays warm for life!