no start due corroded battery harness,how to disconect it on the junction point on 2003 Mercedes-Benz E500

I have strong battery and can not start car .this problem occured a six months ago,mechanic have fixed it but ,it happened again.
please give me advice how to disconect junction point ,and not break it

spray it down with an antirust to help break down the corrosion.

conector have plastic body,hove to oppen it
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I have had the same problem with a key