If you post the model number of the solar panel we can look up if it got reverse diode, wattage and so forth
It might just connect to the battery, fingers crossed
But trickle charging a battery isn't always as straightforward as it first seems
As you have already found out the cigarette lighter isn't live with the ignition off
But there are other considerations
Most are too small, you loose about 50% efficiency behind a windscreen ( rough estimate)
If it's bigger enough to work on a cloudy day, a
200 watt flexible solar panel will boil the battery dry on a sunny day.
If there no diode built in, it will reverse flow and drain the battery at night
All is not lost a MPPT solar controller from China is around £10
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