Electrical connectors/insulators

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Can anyone give me some advice on wire connectors which also insulate.

I have used plastic wire nuts which hold the wires together and also provide some insulation against direct wire-to-wire contact.

When I took apart my strip-light to replace the ballast, the wires are held together by a plastic thingy that seems to hold them really tight and also looks like pretty well a moisture-proof seal. This looks like a rectangular plasting thing which snaps shut and I can't get it open.

Anyone know what these things are and where I can get them, I need to attach three wires together inside my strip light.