If the solar battery charger is weather proof and you can park off road on your own property then the easiest solution is to get yourself a 12V socket on crock clips.
https://www.amazon.co.uk/s?k=12v+ac...V+socket+with+cro,aps,150&ref=nb_sb_ss_i_1_19
If you want a more permanent "in car" power socket then personally I would be looking to run a fused feed directly from the battery junction box into the car AND NOT mess around with the rest of the vehicles wiring.
When I fitted my tow bar and auxilary electrics I took the opportunity to fit a 3 way 12V socket (on a flying lead so can be beneath the boot floor out of the way or velco attached to the boot side panel carpet). If your solar charger has a long lead then putting the 12V power socket in the boot would be my preference as getting a 12V feed into the boot area is quite easy as there is already a grommeted cable path for the reversing sensor wiring.
Have not investigated engine bay to cabin cable path possibilities but do seem to recall from when I fitted my tow bar that potential possibilities were challenging. Could be wrong on this as it was 4 years ago.