I bought this MP4 player, cheap, off ebay some time back. It didn't use an internal built in battery but a removeable cellphone-style battery. So, I removed the battery, and soldered two wires to its contacts. The standard colors, red for positive and black for negative. The solar cell will have to be attached to its back as there is no space for it on the front.
I then soldered a blocking diode to the positive side of a solar cell I had salvaged from a garden light I bought at a dollar store. To make sure it would still work I connected it to a green LED and held it up to a light bulb, and the LED lit up. I have done tests with various-sized solar cells connected to lithium batteries and this one worked the best. I have also tried solar cells taken from solar powered calculators and those that powered solar powered toys and determined that the smaller cells take wayyyyy to long to recharge the battery.
Then I dremeled out two openings in the slide-open back to accommodate the two leads and glued the solar cell to the back.
Then I took the lithium battery I had been using and put it back inside the MP4 player.
After that, I first reconnected all the three positive wires together(The red wires from the MP4, the battery, and the solar cell.) then the three negative wires(The black wires from all three.) then soldered them together and coated them in hot glue as insulation. Then I slid the back back on, and now I have a solar recharged MP4 player.
And here's the back and side view of the completed MP4 player. It still works perfectly and will not run out of power any time soon... I'll add a protective bumper around the solar cell later... When the power runs low all I have to do is put it face down in a sunny window or next to a lamp to recharge it.