You won't get anything meaningful with a multimeter; especially now that everyone uses digital meters, you'll just get flashing changing values.
Best way is to get a known speaker and plug it into the channel to be tested = verify output compared to other channel.
Then plug the speaker to test into a known good channel. In other words, swap them over; see if the fault "moves".
You can test a speaker statically with a multimeter - when tested on ohms, it will "click" - but that only tells you if open circuit, not much else.
That's as technical as you want to get.
I have 4 oscilloscopes here - so I can look at waveforms, check their amplitude and then calculate power.......... but I have never bothered - I "swap them out" as above.
And don't forget to make sure they are connected with the correct polarity. Normally these days either have a one way plug - or pos large, neg small. But if one is opposite of the other, they will "rob" each other and not be as loud.
Because speaker cones are meant to extend and contract in unison. If one extends while the other is contracting, they detract (rob) from each other. THAT is why they are polarity conscious.
Thanks for the help .Problem i have we are trying to hook up a subwoofer and we tapped off the rear speaker wires to the amp and the sub works but not very loud.so only thing i can up with is the signal wires not having correct voltage as per tech at boss audio.
I am an (amongst other things) a radio tech; but most of my amp experience is with older stuff; all the basics are generic (theory); but what they make/offer and "current terms" sometimes change. Analogue amp theory hasn't changed in a hundred years; but the hardware and names have.
Amplifiers have different inputs and are designed to accept (in the case of "add ons") full power that would normally go to a speaker.
Ones that are designed to run from a head unit (the radio/media player) normally take "signal" inputs that are just info; only designed to run an amp/low amplitude - used to be called "component" outputs.
Many general purpose amps will have both so they can be flexible.
Impedance matching is important; if using an impedance mismatch, power will be lost, or it will be distorted.
If the supplier has said is fit for the application; then they should know best as to what matches what.
I am up with the theory - but the "younger guys" on here probably can chime in as they do these mods all the time and know what is current/what most people use.
Sorry if that wasn't of help/made it more confusing.
Did you put the amp INSTEAD of the rear speaker - or bridged across it? Is that what the supplier said to do?
If you put it INSTEAD of the rear speaker - ALL that power will feed the amp.
If you put it across the speaker (parallel) - then (depending on impedance of the amp) HALF the power will go to amp/half to speaker. The speaker should be 8 ohms. If amp input is 8 ohms - that is 4 ohms total (parallel will halve impedance); and lower impedance will not transfer all the power. However; if the impedance of the amp is MORE than 8 ohms - MOST of the power will be across the old speaker and less to the amp.
But if that is what it is designed to do - it should allow for that.
Double check with the supplier to see if meant to be "across" or "instead of" old speaker.
Sorry to delve into the theory of it. Electrics are boring - and if get too technical, people hate it/complain.
But if you DON'T explain it - others complain you "left stuff out."
To be fair, electrics is a terrible subject. Many don't comment as it isn't worth the hassle. I normally keep things to myself, as easier than arguments.........
If you keep it brief, and people misunderstand and damage something, they blame you.
If you explain and take more than three lines "too involved/going on".
And then you get arguments, because "Neither me or any of my mates have heard that, you must be wrong." Honestly? If none of them have any electrical theory training and "haven't heard of their problem" - wouldn't going to an electrical tradesman who HAS, be better?
I was in our auto store the other day and told the 17 year old a fact about batteries (hence why I bought the one I did); he said skeptically "I have never heard THAT before........" I didn't say anything - but thought "you have been working with car stuff for a couple of years MAX - get ready for a LOT you "haven't heard before".........."
Most learn their electrics from mates/common sense. Fair enough, but many draw their own conclusions - and can draw the wrong one. So it pays to check.
Did you get anywhere with substitutes and checks? Any feedback from the supplier????
Not sure of your year but I have a 2012 300c luxury edition. It has the Alpine system in it. I added a sub and amp to my car with ease. Actually with the battery in the trunk it was one of the easiest installs. I used a line out converter to tap into the two rear speakers and the only wire I had to run into the car was the remote wire. Which I ran to the aux port in the backseat. I would say if you are just tapping off the speakers and not getting the volume look for crossed wires. Or make sure the amp is getting the power it needs. Good luck.
With the new amp wiring disconnected, is there a decent output from the rear loudspeakers (i.e. does it sound OK and as loud as the front)? Don't bother with test equipment, use your ears! Use the head unit to pan it front and back to compare.
If it sounds fine then try another amp/sub (what are you using, out of interest?)