I'm not sure the original sealant is silicon based. It could be a type of butyl rubber, in which case a number of solvents could work*. On the glass I would use steel wool soaked in mineral turps or kerosene. Then wash the glass with hot water and detergent, and clean the edge with acetone before the new sealant is applied. Don't use acetone on the plastic housing, use alcohol instead.
*if the sealant is properly cured by chemical cross-linking, it will at most swell a bit but not dissolve. If a sealant is not cross-linked, it will "melt" and go gooey.
I've only resealed one headlight and must say I did a crap job by using a cheap sealant. If I'd used something proper like Sikaflex it would have been ok.
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