If the car is on the drive, then use a proper smart charger (NOT a cheap battery-boiling “trickle” charger) with a suitable extension lead, but only if the charger and its plug+extension socket are small enough to live in the car or under the closed bonnet. It’s how I did the Mazda3 yesterday.
Failing that some of the Solar panels have just about enough output each day to overcome the car’s daily dark current losses. I have one that is just about adequate, but it’s no longer on sale, and there are a lot available of hugely varying quality, some better, many a lot worse! Caveat Emptor. But if you can find a good one then it is worth having when there is no mains near the car.
The easy way is to feed its output into the OBDII socket (on most makes of car after 1996.) This has permanent live connections to the battery, (don’t try the accessory socket because with most cars it’s only on with the ignition). The picture shows the pinout on the OBDII connector inside the car by the driver’s right knee.
The tricky bit is quickly finding a reasonably priced bare plug to go into the OBDII socket.
The solar panel can be connected as indicated;
solder +ve (Red wire) from Solar to OBDII plug pin 16. Ideally this wire should include a fuse, say 2A or 5A.
solder -ve (Black wire) from Solar to OBDII plug pin 4 .