Hi, first of all I would think a way to test if it's the left speaker or the headunit stereo that isn't working would be to play about with the balance settings. Set it all the way to the left, and then to the right. When I decided to replace my stereo a few months ago for more connectivity such as AUX-IN and USB, I noticed my left speaker wasn't working too so I decided to go and replace both of them.
If you need to replace the stereo, then you'll need to take off the center panel off that surrounds the stereo and ventilation dials. That's easily removed via 2 screws under the cigarette tray and another 2 under the stereo. I recently replaced the stock stereo in my Micra K11 with a
Pioneer DEH-2220UB from Amazon which cost about £70. In addition, I had to purchase a connector of the
PC2-13-4 type (hopefully all the connections are still intact and haven't been messed with), and then an aerial adaptor of the
ISO to DIN type (although you may not need this depending on your new stereo). Both cost an additional £10 together.
If you need to replace the speakers, then you need to make sure you get one with a suitable mounting depth, or else you'll need to do a bit of DIY and make a makeshift mount. I bought mine from Amazon, which are some
Pioneer TS-G1711i I bought from Amazon for £30. I was initially thinking component speakers (these are coaxial) but I'm not much of an in-car entertainment enthusiast and these just did the job, and very well too! The mounting depth was about 47mm, and I think the door panel can accept up to 54mm without modifications as mentioned
here.
The easiest way to connect your ipod would be via an AUX-IN cable such as
these providing your stereo has one of the sockets which most should have these days.
Here are threads that I made when I needed help installing my stereo and speakers:
Which ISO connector for my Micra S 5-door K11 (with pictures)
What do I need to connect these speakers to the front door panel?
I hope this helps!