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I think first of all, on a non-electric or hybrid car, 100W panel might be too much. It's not enough to shut off the alternator completely for a long time, but it will quickly fill the battery.
If you just have solar panels laying around, I think an approach could be:
1. Replace the lead acid battery with a higher voltage and higher capacity lithium ion battery. The idea is that the alternator should not be able to charge the battery all the way to full, and it should not be applying excess voltage that would normally just create hydrogen and oxygen in a lead acid battery.
LiFePO4 is a bit problematic because its nominal voltage is a little lower. The usual lithium polymers will be around 50% charged at 14.8v at the alternator which IMO is about perfect. If you want something safer, you can use 6S lithium titanate which can charge up to 18V.
2. Limit the solar charging to 16V, I don't think car electronics are supposed to take much more than 16V.
3. If you can, limit the alternator charge voltage to 14V.
The result will be that you end up with a half full battery while driving, the solar panel charges the battery up to full if it's sitting outside, and you start off with a battery voltage high enough that the alternator won't be doing work.
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