we all know the 26 AWG wires that come inside UTP cables. Those surely are flexible. But what a bout an AWG 15-16 such copper wire, is it flexible enough to use as feeding cable for the 12 V car lighter socket ?
The problem with solid core wires is they are prone to breakage through repeated mechanical flexing. If your application has any moment of the cable, the wire could break after installation.
If you're fine with the risk of breakage than use it.
On a side note Solid UTP cables should not be used for the connection between your computer and the wall for this reason, only use stranded UTP cables. Walking on solid core UTP cables is also a great way to break them.
Another 'nother side not, as with all cables, make sure you properly crimp and strip the ends off of wires, any knicks or improper crimping can also fatigue the wire and cause it to break in the future.
This manual is a great resource for tips on how to handle wiring correctly. Student Workbook for Crimp, Cable and Harnessing